Stir together the milk and egg. Add to biscuit mix; stir just until moistened. Add the onion mixture, half of the cheese, and half of the poppy seed to biscuit mix mixture. Spread batter into prepared pan. Sprinkle with the remaining cheese and the remaining poppy seed. If desired, arrange cooked, thinly sliced onion on top.
Drizzle the 1 tablespoon melted margarine or butter over mixture in pan. Bake in a 400 degree F oven about 20 minutes or until a wooden toothpick inserted near the center comes out clean. Serve warm. Makes 8 servings.