Begin by making the tart dough: In a large bowl, sift together the flour, sugar and salt. using a pastry cutter, cut the butter and sour cream into the flour, until the flour is fully incorporated, and everything is the size of peas. Add the ice water, starting with one tablespoon. Stir it in with a fork. You may not need all the water. Just stir in enough water, until a scraggly dough is formed. Empty the dough onto a floured board and shape the dough into a disc. Wrap it in plastic wrap, and refrigerate for 1½ hours.
On floured parchment paper, roll out the disc of dough, covered with more parchment paper, into a 15-inch round. Place on a baking sheet and refrigerate for one hour.
Preheat the oven to 400 degrees. Mix together the berries and sugar and place in the center of the dough, leaving a 2-inch bare border. Use the parchment to lift up a three-inch portion of the dough and fold it over the fruit. Repeat with the rest of the dough to create pleats. Brush the pleated dough with one egg mixed with one tablespoon of water. Sprinkle with the sugar and bake for 30 minutes or until the crust is golden brown. Let cool completely on a wire rack.