HEAT oven to 400°F.
BROWN meat in large nonstick skillet on medium-high heat; drain. Return meat to skillet. Add onions; cook and stir 5 min. Stir in soup. Spoon into 2-qt. baking dish sprayed with cooking spray.
WHISK egg and milk in medium bowl until blended. Stir in cheese and onions. Add baking mix; stir just until moistened. Drop by spoonfuls over meat mixture.
BAKE 35 min. or until biscuit topping is golden brown.