Step 1Combine the flour, three-fourths teaspoon salt and nutmeg. Add the two-thirds cup of butter pieces. Use the tip of a knife to scrape the vanilla bean seeds from the bean onto the butter. Use a pastry blender or your hands to work the butter into the dry ingredients until just small pieces of butter remain and the flour resembles coarse-ground meal.
Step 2Sprinkle the ice water over the flour mixture and stir with a fork until the dough holds together and forms a ball. Place the dough on a lightly floured surface and form the dough into two flat rounds. Wrap the dough in plastic wrap and refrigerate about 1 hour or until firm.
Step 3Heat the oven to 400 degrees. In a large bowl, toss together the blackberries, raspberries and blueberries with the Muscat. Allow to macerate for 5 minutes.
Step 4In a small bowl, combine the sugar, tapioca and remaining one-eighth teaspoon salt. Pour the mixture over the fruit, tossing to coat the fruit evenly. Let the fruit stand 20 to 25 minutes while rolling out the crust; if using cornstarch to thicken the mixture, the fruit needs to stand only 5 minutes.
Step 5Remove the dough from the refrigerator and allow to sit out for 10 to 15 minutes. Roll out one portion of the dough on a lightly floured surface into a 12-inch circle, about one-eighth-inch thick. Roll the dough loosely around the rolling pin and carefully unroll it onto a 9-inch deep-dish pie plate. Gently press dough onto the bottom and sides of the pie plate, leaving excess dough hanging over the edge.
Step 6Roll out the second round of dough into a 13-inch circle. Cut a 4-inch circle out of the center. Use a 1 1/4 -inch star cutter to cut stars out of the dough around the cut-out circle; additionally, cut stars out of the 4-inch circle that has been removed from the dough. Set aside cut-out stars to decorate the edge of the pie.
Step 7Fill the pie shell with the fruit and juices. Sprinkle the remaining 1 tablespoon of the cut-up butter over the top of the pie. Roll the dough loosely around the rolling pin and carefully lift the dough and unroll it over the pie.
Step 8Trim the edges, leaving a 1-inch edge. Fold the edges under to fit the pie plate, gently pressing it flat on the rim of the pie plate. Brush the crust with beaten egg.
Step 9Press the stars around the edge of the pie plate. Brush the stars with the beaten egg. Sprinkle with the Demerara sugar. Put the pie on a baking sheet and bake 55 to 60 minutes, until it is golden brown and bubbly. If the pie browns too quickly while baking, loosely tent the top of the pie with foil. Let the pie cool on a wire rack to warm.
Step 10Whip the creme fraiche until light and fluffy, about 2 to 3 minutes. Stir in powdered sugar.
Step 11Cut the pie into serving-size pieces. Serve with a dollop of creme fraiche on top of each slice.