Preheat oven to 375F/190C. Lightly spray a 2 1/2 qr/2,5L (11x7-inch is good) baking dish.
Cook elbows per package direction; drain and set aside.
Brown ground beef and onion in a large skillet over medium-high heat. Stir in soup, salt, pepper and garlic powder. Check for seasoning and add more if necessary.
Add cooked elbows, mozzarella and 1 cup of the cheddar; stir to combine. Turn into prepared pan, cover with foil and bake for 25-30 minutes.
Remove foil, top with remaining cheese and bake, uncovered, for 10 minutes or until cheese is melted and bubbly.