Lemon Rhubarb Bars

A sweet and tart lemon custard envelops a generous amount of vibrantly hued rhubarb. With a buttery shortbread base and a decorative toasted meringue topping, these lemon rhubarb bars are an excellent way to celebrate the arrival of rhubarb in our gardens and local shops.

LEMON RHUBARB BARS WITH MERINGUE TOPPING

I've been eagerly awaiting the arrival of rhubarb to my local grocery stores. Just as citrus fruits signal the shift of seasons into winter, rhubarb is firmly stuck in my mind as one of the first signs that spring is truly on its way. Around the same time that the flowering fruit trees burst into colorful cotton candy blooms, those first brightly pink and red hued stalks appear. The growing season for rhubarb is brief, so I try to take advantage whenever I see it. I've used it in cakes, tarts and pies - including this rhubarb frangipane tart and a strawberry lime rhubarb slab pie.

Here I wanted to incorporate the rhubarb into a recipe for shortbread bars, and took inspiration from an old fashioned dessert favorite, rhubarb custard dessert bars, but with a lemony custard filling (and no mysterious "whipped toppings.") Not quite a lemon bar, nor a rhubarb custard bar, I've married the two by adding three types of lemon (freshly squeezed lemon juice, lemon zest and lemon oil) and a generous amount of diced rhubarb. The origin matters less than the end result: a vibrant, brightly flavored bar packed with a balance of sweet custard, rich shortbread crust, and the inevitable zing from tart lemon and rhubarb combined. It's all tempered with a sweet decorative layer of toasted Swiss meringue that ties it all together nicely.

INGREDIENTS AND RECIPE NOTES

UNSALTED BUTTER

Using unsalted butter allows you to control the total amount of salt in the recipe. With salted butter, brands use different amounts of salt, so you’re never sure where you’re starting with salt quantity. With a simple recipe for bars that only has 4 ingredients in the base, each one really counts. Too much salt could throw off the final taste, so I always recommend using unsalted butter when specified.

LEMON OIL

I use lemon oil in this recipe to add an extra boost of lemon flavor to the custard filling. Lemon oil is concentrated and really helps achieve a powerful tang. I find it to be much more flavorful than lemon extract.

SWISS MERINGUE

Making a Swiss meringue requires a bit more effort than common meringue, but it has a stronger structure (and pasteurization!) which helps keep the meringue stable over a longer period of time. Although these bars are really best soon after making them, I stored extra meringue topped bars in the fridge for several days and they held up just fine with no weeping. To top the bars, you can either spread a full layer of meringue onto the top, or opt to pipe meringue decorations on each bar as I did.

If you spread meringue to cover the entire top, you will miss showing off the vibrant red and pink hues of the rhubarb, but you'll gain the sweet taste of meringue with every bite of lemon rhubarb bar. The next time I make them, I'll consider more meringue as it's such a delicious pairing.

If you pipe the meringue on, use a 16" (40cm) piping bag with a decorative tip. A St. Honore tip would be ideal for the zig zag pattern that I piped. I didn't have one and ended up using an Ateco 366 leaf tip with the "beak" held sideways, perpendicular to the bars. Be creative with the piping - most large or jumbo tips with a wide enough opening will work well for meringue. If you choose to pipe decorative meringue patterns, you will most likely end up with some leftover meringue. To avoid food waste, consider making piped meringue kisses (pipe onto parchment and cook in a 200 degree F (93 degrees C) oven for 2 to 2 ½ hours).

FREQUENTLY ASKED QUESTIONS

CAN I FREEZE THESE LEMON RHUBARB BARS?

These bars are best when eaten fresh. Most dessert recipes that have a custard element do not recommend freezing after they have been baked,. They tend to separate and turn watery. For best results, eat these bars fresh and store any extra in the fridge for up to 5 days in a single layer airtight container.

HOW SHOULD I PREPARE THE RHUBARB?

Prepare your rhubarb by rinsing the stalks in water to ensure all dirt is removed. Trim stalks by removing any remaining leafy green portion. Rhubarb leaves can be toxic in large amounts as they contain high amounts of oxalic acid (though you'd have to consume a significant amount to feel the ill effects). If the stalks are thicker than your thumb in width, split them lengthwise. I found early forced rhubarb (you can read more about forced rhubarb vs. regular rhubarb here) which tends to be smaller in diameter, so I didn't need to split mine. Dice them into even pieces about the size of your thumbnail.

MORE RHUBARB RECIPES

• Rhubarb Frangipane Tart
• Strawberry Lime Rhubarb Slab Pie
• Cherry Rhubarb Pie
• Brown Sugar Cake with Rhubarb Compote and Creme Fraiche Buttercream
• Rhubarb Apple Pie with Ginger and Lemongrass

MORE BAR COOKIE RECIPES

• Caramel Pecan Shortbread Bars
• No Bake Carrot Cake Bars
• Blood Orange Cornmeal Shortbread Cookies
• No Bake Key Lime Cheesecake Bars
• Butterscotch Dark Chocolate Bars

Lemon Rhubarb Bars

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Refreshingly tart rhubarb enveloped in a sweet lemony custard layer with a buttery shortbread base. Topped with toasted Swiss meringue, these bars are an excellent way to highlight the vibrant pink color and unique flavor of rhubarb as it makes its all-too-brief appearance in spring.

Prep time20 minutes

Cook time45 minutes

Total time1 hour 5 minutes

Servings16 bars (8" square pan)

INGREDIENTS

SHORTBREAD CRUST

• ½ cup (113g, 4oz) cold unsalted butter, cut into cubes
• ¼ cup (50g) granulated sugar
• 1 cup (120g) all-purpose flour
• ¼ teaspoon kosher salt

LEMON RHUBARB CUSTARD LAYER

• 3 Tbsp (23g) all purpose flour
• ¾ cup (150g) granulated sugar
• 2 large eggs, beaten
• ¼ cup (60mL) heavy whipping cream
• zest of 1 lemon
• ¼ cup (60mL) lemon juice, freshly squeezed (approx. 1 large lemon)
• ½ teaspoon lemon oil or extract
• 3 cups (366g, 0.8lb) rhubarb, diced

SWISS MERINGUE TOPPING

• 3 egg whites (approx 100g)
• ¾ cup (150g) granulated sugar
• ¼ teaspoon vanilla extract

INSTRUCTIONS

SHORTBREAD LAYER

• Preheat the oven to 350° F (177 C, gas mark 4).
• Line an 8" (20 cm) square pan with parchment paper, extending a flap of parchment over opposite sides as a handle to aid in removing the bars later.
• In the bowl of a stand mixer fitted with the paddle attachment, combine the butter, sugar, flour, and salt and mix until the dough begins to come together with a lumpy, sandy texture.
• Dump the dough into the prepared pan and press with your fingers until it is spread evenly into all areas of the pan. Cover with plastic wrap and use the flat base of a drinking glass or the bottom of a measuring cup to smooth the top and firmly pack down the dough. Chill in the fridge for 30 minutes.
• Bake the shortbread for 10-12 minutes, until top has gone from shiny to dull, shortbread is just baked through, barely starting to turn golden brown, and beginning to smell like toasted butter. Remove from oven and set aside to cool on a baking rack.

LEMON RHUBARB CUSTARD LAYER

• In a medium size bowl, whisk together flour and sugar. Add eggs and whisk to combine. Add cream, lemon juice, lemon zest and lemon extract and mix until completely blended. With a spatula, fold in chopped rhubarb and pour custard mix onto cooled shortbread crust. Rap pan firmly on countertop several times to remove air bubbles that may have formed in custard.
• Bake at 350° F (177C, gas mark 4) for 40-45 minutes, until custard layer is completely set - it should not jiggle. Cool completely on a rack. Once cool, place in refrigerator to set for at least one hour.

SWISS MERINGUE TOPPING

• In a small (1 qt ) saucepan, whisk together egg whites and sugar. Heat over low heat, whisking frequently, until mixture is warm and granulated sugar has dissolved. Rub some of the egg white mixture between your fingers (carefully - it might be hot). If you feel sugar granules, keep mixing until it feels smooth. If you have a thermometer, it should read 160° F (177 C).
• Transfer warm egg mixture to the bowl of a stand mixer fitted with the whisk attachment. Beat until stiff peaks forms and mixture has a glossy sheen - about 5 minutes. The meringue should be very thick and hold its shape (no drooping) when the whisk is dipped into the mixture and turned upright.
• To top the bars, you can either spread a full layer of meringue onto the top, or opt to pipe meringue decorations on each bar as I did. Remove fully chilled bars from pan. Place bars onto a metal baking sheet, removing the parchment paper from underneath (kitchen torches and paper do not mix!)
• To spread meringue over the bars: Scoop the meringue out onto the baked custard and gently spread evenly to cover the filling, making swirls with an offset spatula or spoon. Toast the meringue using a kitchen torch. Slice into 16 squares, wiping the knife clean in between each cut.
• To pipe meringue onto the bars: Slice into 16 squares, wiping the knife clean in between each cut. Prepare a 16" (40cm) piping bag with a decorative tip. A St. Honore tip would be ideal for the zig zag pattern that I piped. I didn't have one and ended up using an Ateco 366 leaf tip with the "beak" held sideways, perpendicular to the bars. Be creative with the piping - most large or jumbo tips with a wide enough opening will work well for meringue. Toast the meringue using a kitchen torch.
• Serve immediately. Keep extra bars refrigerated in a single layer in an airtight container.

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MORE RHUBARB DESSERTS

If you liked these creamy lemon squares packed with rhubarb, you might also enjoy these rhubarb recipes:

• Rhubarb Frangipane Tart
• Cherry Rhubarb Pie
• Strawberry Rhubarb Panna Cotta Tart
• Strawberry Rhubarb Ice Cream Bars
• Strawberry Lime Rhubarb Slab Pie
• Rhubarb Apple Pie with Ginger and Lemongrass
• Brown Sugar Cake with Rhubarb Compote and Creme Fraiche Buttercream