Preheat the oven to 350°F (175°C).
In a large skillet over medium heat, brown the ground beef and chopped onion until fully cooked and onions are translucent. Drain any excess fat.
In a mixing bowl, combine the cooked beef, cream of mushroom soup, milk, garlic powder, salt, and black pepper. Mix well until all ingredients are thoroughly combined.
Pour the beef mixture into a 9x13 inch baking dish, spreading it evenly across the bottom.
Sprinkle the shredded cheddar cheese evenly over the beef mixture.
Layer the frozen tater tots on top of the cheese, arranging them in a single even layer.
Bake in the preheated oven for 30 minutes, or until the tater tots are golden brown and crispy.
Once baked, let the casserole cool for a few minutes before serving.