In France, rhubarb appears fleetingly in the markets in May and June, just in time to be paired with gariguette or charlotte strawberries, before disappearing for another year. In this dish however, rhubarb is the sole star, the shining tart crown atop a warm pastry crust.

Both green and red rhubarb can of course be used for this dish, but it's the red varieties that really leaves an impression visually if cooked properly. So often it can lose its gorgeous ruby hue as it cooks but the trick is not to precook the stems and instead bake them in extra-thin slices, so they cook quickly and retain their shape and bright red color.

Ruby Rhubarb Bars

Equipment: A 9 1/2  x 9 1/2-inch (24 x 24 cm) baking pan   |   baking parchment   |   a food processor.

Pastry
4 tablespoons (60 g) salted butter, chilled
1 cup (140 g) unbleached, all-purpose flour
1/3 cup (65 g) confectioners’ sugar
Grated zest of 1 lemon, preferably organic
1/2 teaspoon fine sea salt   
2 tablespoons plain nonfat yogurt

Topping:
3/4 cup (150 g) unrefined cane sugar, preferably organic, vanilla scented (see Note)
1/4 cup (40 g) unbleached, all-purpose flour
4 large egg whites, preferably organic and free range
3 cups (300 g) thinly sliced red rhubarb stalks, (about seven 10-inch; 26 cm)

1.    Center a rack in the oven. Preheat the oven to 400°F (200°C).
 
2.    Line the baking pan with two pieces of baking parchment, letting the parchment hang over the sides. (This will make it easier to remove the dessert once baked.)

3.    Prepare the pastry: In the food processor, combine all the pastry ingredients and process to blend. The mixture should be soft and pliable.

4.    Press the dough evenly into the bottom of the baking pan. Place the pan in the oven and bake until firm, about 12 minutes.

5.    While the pastry is baking, prepare the topping: In a bowl, combine the sugar, flour, and egg whites and whisk to blend. Add the rhubarb and stir to coat it evenly with the egg-white mixture.

6.    Remove the pan from the oven and spoon the rhubarb mixture over the warm pastry. Return the baking pan to the oven and bake until the topping is firm and golden, 35 to 40 minutes. Remove from the oven and let cool. Remove from the pan and cut into 16 squares. Serve at room temperature. (Store in an airtight container at room temperature for up to 3 days.)

Note: to make vanilla-scented sugar: Flatten 1 or several moist vanilla beans. Cut them in half lengthwise. With a small spoon, scrape out the seeds and place them in a small jar; reserve the seeds for another use. Fully dry the vanilla bean halves at room temperature. Place the dry halves in a large jar with a lid, and cover them with sugar. Tighten the lid and store for several weeks to scent and flavor the sugar. Use in place of regular sugar when preparing desserts.