PREHEAT 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-L baking dish.

BEAT egg and milk in medium bowl with wire whisk. Add cheese and onions; mix well. Add baking mix; stir just until moistened. Drop by spoonfuls over meat mixture.

BAKE 35 min. or until biscuits are golden brown and cooked through.