In a food processor, add flour, cornmeal, salt, sugar and lemon zest; pulse to combine. Add butter and pulse until the mixture resembles a coarse meal. Use a pastry cutter instead of a food processor, if desired. While pulsing, slowly add water until the dough comes together, approx. 12 pulses. Do not over mix. Turn the dough onto a piece of plastic wrap, form a 2-inch thick flat disc and wrap. Refrigerate at least 1 hour, up to overnight. If the dough has been refrigerated for more than 2 hours, place onto a work surface and allow to soften, approx. 10-15 min.
Preheat oven to 375 degrees, placing oven rack in the middle.
Lightly flour rolling pin, work surface and dough disk. Roll dough out, to create a 12-inch round, approx. 1/4-inch thick. Line sheet pan with parchment paper and transfer dough to parchment. In a mixing bowl, gently toss together nectarine wedges, sugar, salt, lemon juice and cornstarch. Leaving 2-inch border, arrange 1/4 c blackberries atop dough round, then top with nectarines. Brush border with beaten egg. Working quickly, fold the edges of the dough over the fruit, creating overlapping pleats, pinching gently so folds adhere. Brush outer edge of dough with egg and sprinkle with sugar.
Bake approx. 50 min, top with remaining 1/4 c blackberries and bake 10 min more, until crust is golden brown and fruit is bubbly. Let cool at least 1 hour. Serve warm or at room temp.
*Peaches, plums or apricots and various berries can be substituted. The fruit should be ripe but firm. Add 2-4 T additional sugar if using plums due to their tartness.