This dessert takes a humble summer berry and transforms it into an elegant dessert. A poppy seed-dotted crust is the bed for a rich crème fraîche custard and sweet blueberry filling. Topped with fresh blueberries, it’s a beautiful and luxurious way to present this popular summer berry.
Preheat the oven to 350 degrees.
Coat a 10-inch loose-bottomed fluted tart pan with nonstick spray and set it aside.
In the bowl of a mixer with flat paddle on medium-high speed, beat the butter until it is creamy, about 1 minute. Gradually add the sugar and continue to beat at medium-high speed until lightened and creamy, about 2 minutes. Beat in the egg yolk until it is well combined. Add the flour and poppy seeds and pulse the mixer on and off until it begins to combine, and then run the mixer on medium-low speed just until dough begins to form. Scrape the dough out onto plastic wrap and use the wrap to help press it into a round disk. Wrap the dough thoroughly. Refrigerate the dough for at least 2 hours or overnight.
On a lightly floured surface, roll the refridgerated dough out to a 13-inch round and fit into the pan, pressing it into corners and trimming the top. Refrigerate the pressed dough for at least 2 hours or place it into the freezer for 15 minutes while the oven preheats. At this point, you can double wrap the tart shell very well in plastic wrap and then aluminum foil and refrigerate up to 2 days before baking, or freeze it for up to 1 week.
Position a rack in the middle of theh oven. For a partially baked tart shell, bake it directly on the oven rack for about 8-12 minutes or just until it feels dry to the touch and is beginning to color around edges. Check about halfway through baking; if the crust is puffing up, gently press it back down with back of a fork.
Allow the crust to cool completely on a rack.
In a small saucepan over medium-high heat, combine the blueberries, sugar, and water. Bring them to a boil, stirring often. As soon as the mixture looks juicy, use a potato masher to crush the berries. Continue to boil the mixture, stirring often, for about 6-8 minutes or until the berries have broken down and the mixture has thickened slightly.
Allow the mixture to cool completely, stirring occasionally to release heat.
Scrape the crème fraîche into a small saucepan. Whisk it gently to loosen it and heat over low heat until warm, whisking occasionally. Keep it warm and set it aside.
In a medium bowl, whisk the egg and sugar together. Slowly whisk in crème fraiche until custard is smooth.
Spread blueberry filling evenly over bottom of par-baked tart crust, and then slowly pour the custard over the fruit. Some of the fruit might come up through the custard creating a marbled effect; that is fine. Bake about 15-20 minutes or just until the custard is set. It should lightly jiggle in the center when you gently shake the pan, but will be firmer around the edges.
Allow the pan to cool pan completely on a rack, then refrigerate it until it is firm, at least 6 hours or overnight.
Place blueberries in a heat-proof bowl. In a small saucepan, stir sugar and cornstarch together. Over medium heat, whisk in water and lemon juice and bring to a boil. Whisk often and boil until mixture is clear, about 30 seconds to 1 minute. Immediately scrape this mixture over the blueberries and toss gently to coat. Quickly and gently scoop berries out of the bowl and arrange them in an even layer over the chilled tart.
Refrigerate the tart for up to 4 hours. Allow it to sit at room temperature for about 15 minutes before serving.