In a large mixing bowl, combine flour and salt. With a pastry blender or 2 knives, cut in the chilled butter until the mixture resembles coarse crumbs. Add the egg and vinegar. Slowly add the water, as needed, mixing to form the dough, until well blended. Turn the dough out onto a sheet of plastic wrap and knead gently until the dough becomes smooth, only a few seconds. Wrap well and chill for 30 minutes.
Saute onion in butter and add spinach over medium heat. Cool and then combine with cheese, hazelnuts, and raisins.
On a lightly floured surface, roll half the chilled dough to 1/8 inch thick (keep the other half refrigerated until ready to use). Cut out 3-inch circles. Spoon about 1 tablespoon of the filling into the center of each circle. Brush egg wash around the edges of the dough. Fold the edges up forming a pouch, and seal.
Refrigerate the empanadas on greased or parchment lined baking sheets at least 30 minutes.
Preheat oven to 375 degrees F.
Brush the empanadas with egg wash and bake until golden brown, about 25 minutes.