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; cook 3 min. or until heated through. Spoon into 2-qt. baking dish.
BEAT egg and milk with whisk in medium bowl. Add cheese and onions; mix well. Add baking mix; stir just until moistened. Drop by spoonfuls over meat mixture.
BAKE 35 min. or until biscuit topping is golden brown and cooked through.