Combine ingredients: Add the cooked pasta shells, tomato soup, and 1/2 cup of water to the skillet with the browned ground beef. Stir well.
Add half the cheese: Add half of the shredded cheddar cheese and half of the mozzarella cheese to the skillet. Stir and cook for 2-3 minutes over medium heat, until the cheese is partially melted.
Assemble the casserole: Pour the mixture into the greased 9×13-inch baking dish.
Top with remaining cheese: Top the casserole with the remaining shredded cheddar cheese and the remaining mozzarella cheese.
Bake: Cover the baking dish with aluminum foil and bake for 20 minutes.
Uncover and finish baking: Remove the foil and continue baking for an additional 10 minutes, or until the cheese is melted and bubbly.
Broil (optional): For a browned cheese topping, turn on the broiler for the last 2-3 minutes of baking, watching carefully to prevent burning.
Serve: Serve the Hamburger Casserole immediately. Store any leftovers in the refrigerator for up to 3 days.
