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

Beat butter in bowl of an electric mixer fitted with paddle attachment on medium-high speed until creamy, 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 pulse the mixer on and off until it begins to combine. 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 dough thoroughly. Refrigerate 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 at least 2 hours or freeze 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. Bake directly on oven rack for about 15 to 20 minutes or just until it is very light golden brown. Check about halfway through baking; if crust is puffing up, gently press back down with back of a fork. Cool completely on rack

Fill a large bowl with ice water and set aside. Whisk egg yolks and Champagne together in top of a double boiler (or deep-bowl for a makeshift double boiler). Whisk in sugar. Set over boiling water that just touches the bottom of the bowl and whisk constantly until very thick and almost tripled in volume. The mixture should form a ribbon when you lift the whisk; this will take about 5 to 8 minutes. Remove from heat and immediately set over bowl of ice water. Whisk mixture until it is completely cool.

Whip cream in a separate, clean bowl until soft peaks form, them fold into egg mixture. Sabayon may be used immediately, or cover bowl with plastic wrap and refrigerate up to 8 hours. If it separates at all, simply whisk before using. You might lose a little volume, but it will be fine to use.

Serve wedges of tart with a generous dollop of sabayon partially on top, partially to the side.

Fill crust with berries, all perched upright, nestled closely together and filling entire crust. Chop remaining berries and place in a medium saucepan; stir in sugar and lemon juice and allow to sit about 5 minutes or until juices begin to exude. Mash berries with a potato masher and bring to a simmer over medium-high heat, stirring mixture occasionally as it cooks. Cook about 3 to 5 minutes or until juices begin to thicken and deepen in color. Immediately scrape mixture into a strainer set over a clean bowl (or large measuring cup if you have it). Press as much liquid as possible through the strainer; discard solids.
Measure liquid out to 1 cup adding water if necessary and pour into clean, small saucepan. Stir cornstarch and water together in a small bowl, and then stir into juice. Bring to a simmer over medium-high heat, whisking frequently, until juice thickens and becomes shiny. Glaze berries immediately, using both a teaspoon and a pastry brush to fill gaps between berries and cover tops and sides of fruit. Sprinkle with nuts, if desired, while glaze is still moist. Chill until ready to serve. Tart may be served immediately or refrigerated up to 6 hours.