Beat the egg and soy sauce in a small bowl, until well blended. Spread about 1 1/2 tablespoons of the egg mixture over one side of each of the 4 of the tortillas, sprinkle evenly with onions. Cover each prepared tortilla with another tortilla, pressing the edges together. Place the tortilla stacks onto 2 large baking sheets. Brush both sides generously with butter. Bake in a 400ºF. oven  for 15 minutes or until golden brown. Cut each stack into quarters and serve hot. Makes 16 appetizers