Prepare pastry dough. Combine flour, sugar, salt and lemon zest in a large bowl. Cut cold butter into small pieces and toss with mixture, working butter in with fingertips until only small pieces remain. Toss with egg yolk and ice water and stir just until the dough comes together.
5 cups all purpose flour, ¼ cup sugar, ½ teaspoon lemon zest, 2 teaspoons salt, 1 pound unsalted butter, 1 large egg, ½ cup ice water
Divide dough into two large pieces and place each on a piece of parchment paper the size of a ½ size sheet pan, or whatever size sheet pans you have. Top with another similar size piece of parchment and roll each piece of lemon pie dough to fit the parchment. Transfer both pieces of parchment lined dough to sheet pans and chill until firm, at least 30 minutes.
Meanwhile, gently stir together filling ingredients.
4 cups fresh blackberries, 2 cups fresh blueberries, 2 cups sliced nectarines, 1 ¼ cups granulated sugar, ⅔ cup all purpose flour, 1 teaspoon lemon juice, ¼ teaspoon salt
Once chilled remove parchment from the top of one rectangle of dough and decorate as desired. Then remove the top of the second dough and gently press into the corners of the sheet pan to form the bottom crust. Top with filling and place decorated dough directly onto filling, parchment paper facing up. Gently peel off parchment and press dough together around the edges of pan to lightly seal using dough scraps to fill in where needed.
Brush with heavy cream or milk then whip reserved egg white until light, fluffy and almost meringue like. Brush fluffy egg white around the crust in random spots then sprinkle coarse sugar evenly over top.
2 tablespoons heavy cream, ¼ cup coarse sugar
Bake until crust is browned and filling is bubbly, 40-55 minutes.