Preheat oven to 350°F (175°C).

Brown ground beef in a skillet over medium heat.

Add chopped onion to the skillet and cook until softened.

Drain any excess grease from the beef and onion mixture.

Transfer the beef and onion mixture to a baking dish.

Spread cream of mushroom soup evenly over the ground beef mixture.

Arrange sliced American cheese on top of the soup.

Cover the cheese with a layer of Tater Tots.

Bake in the preheated oven for 30 minutes, or until the Tater Tots are golden brown and crispy and the casserole is heated through.