In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Stir in tomato sauce and spaghetti sauce mix. Reduce heat; simmer, uncovered, for 5 minutes. Remove from the heat; stir in sour cream. Spoon into a greased 13-in. x 9-in. baking dish. Sprinkle with mozzarella cheese.
Unroll crescent dough into one rectangle; seal seams and perforations. Place over cheese. Brush with butter and sprinkle with Parmesan. Bake, uncovered, at 375° for 25-30 minutes or until golden brown.