In a large bowl, combine the flour and 1/4 cup of the sugar. Add the butter and working quickly and using a pastry blender, two knives,or you fingertips, cut the butter into the flour until the mixture resembles coarse crumbs. Add the sour cream and work in quickly. Knead the pastry very briefly with the heels on your hands. Shape into a ball, wrap in plastic and refrigerate for 1 hour. Preheat the oven to 300 degrees. Divide the pastry in half. Roll out each half between 2 sheets of waxed paper to form a 15x10 inch rectangle. Remove the top piece of waxed paper from each pastry and carefully flip each onto an ungreased baking sheet. Remove the remaining waxed paper. Prick the pastry all over with a fork and bake until just golden about 30 minutes. Allow to cool for 5 minutes, then spread one of the pastry sheets wvenly with the preserves. Carefully cover with the second pastry sheet. Whisk together the remaining sugar and lemon juice. With a teaspoon, add just enough cold water to make a sprea dable glaze. Spread the glaze over the pastry with a spatula as evenly as you can. Allow the glaze to set. With a sharp knife, cut the pastry into neat diamond shapes.