Preheat your oven to 350°F (175°C). This ensures that your casserole cooks evenly and comes out hot and bubbly.
In a large mixing bowl, combine 1 lb ground beef, 1 small onion, chopped, 1 cup breadcrumbs, 1 egg, 1 tsp garlic powder, 1 tsp salt, and 1/2 tsp black pepper. Mix everything together until well blended; this will create a flavorful meat mixture.
Transfer the meat mixture into a greased 9x13-inch baking dish. Press it down evenly to form a solid base for your casserole.
Spread 1 can (15 oz) tomato sauce over the meat mixture. This step adds moisture and flavor, so spread it evenly to cover the entire surface.
Sprinkle 1 cup shredded cheddar cheese on top of the tomato sauce layer, ensuring even coverage for a perfect cheese melt.
Arrange 2 cups tater tots on top of the cheese layer. This adds a wonderful crunch and texture to the dish.
Bake in the preheated oven for 30 minutes. You'll know it's time to check when your kitchen starts smelling amazing!
After 30 minutes, remove the casserole from the oven and sprinkle with 1/2 cup shredded mozzarella cheese. Return it to the oven and bake for an additional 10 minutes, or until the cheese is melted and bubbly.
Once done, let the casserole cool for a few minutes before serving. This will help set the layers and make it easier to slice.