Blend egg and milk with biscuit mix and chopped parsley. Turn dough mixture into greased 9-inch square pan. Sauté onions in butter until tender and lightly browned. Spread onions on dough in baking pan. Whisk egg with sour cream; season with salt and pepper then spread over onions. Bake onion squares at 400° for 25 minutes. Cut into squares, sprinkle with a little paprika, and serve hot.