Coat a 10-inch loose-bottomed fluted tart pan with nonstick spray; set aside.

Beat butter in mixer's bowl until creamy with flat paddle on medium-high speed, about 1 minute. Add sugar gradually and continue to beat, about 2 minutes at medium-high speed, until lightened and creamy. Beat in egg yolk until well combined. Add flour and poppy seeds and pulse the mixer on and off until it begins to combine, and then run mixer on medium-low speed just until dough begins to form. Scrape dough out onto plastic wrap and use wrap to help press into a round disk. Wrap the dough thoroughly. Refrigerate for at least 2 hours or overnight.

Roll dough out on lightly floured surface to a 13-inch round and fit into pan, pressing into corners and trimming top. Refrigerate for at least 2 hours or freeze for 15 minutes while 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 up to 1 week.

Position rack in middle of oven. Preheat oven to 350°F . For a partially baked tart shell, bake directly on oven rack for about 8 to 12 minutes or just until it feels dry to the touch and is beginning to color around edges. Check about halfway through baking; if crust is puffing up, gently press back down with back of a fork. Cool completely on rack.

Combine blueberries, sugar and water in a small saucepan and bring to a boil over medium-high heat, stirring often. As soon as the mixture looks juicy, use a potato masher to crush the berries. Continue to boil, stirring often, for about 6 to 8 minutes or until berries have broken down and mixture has thickened slightly. Cool completely, stirring occasionally to release heat.

Scrape crème fraiche into small saucepan, whisk to loosen gently and heat over low heat until warm, whisking occasionally. Keep warm and set aside. Whisk egg and sugar together in a medium bowl. Slowly whisk in crème fraiche until custard is smooth. Spread blueberry filling evenly over bottom of par-baked tart crust, and then slowly pour custard over fruit. Some of the fruit might come up through the custard creating a marbled effect; that is fine. Bake about 15 to 20 minutes or just until custard is set. It should lightly jiggle in the center when you gently shake the pan, but will be firmer around the edges. Cool pan completely on rack, then refrigerate until firm, at least 6 hours or overnight.

Place blueberries in a heat-proof bowl. Stir sugar and cornstarch together in a small saucepan. Whisk in water and lemon juice and bring to a boil over medium heat. Whisk often and boil until mixture is clear, about 30 seconds to 1 minute. Immediately scrape mixture over blueberries and toss gently to coat. Quickly and gently scoop berries out of bowl and arrange in an even layer over chilled tart. Refrigerate up to 4 hours. Allow to sit at room temperature for about 15 minutes before serving.