Preheat the oven to 375°F. In a large skillet over medium heat, cook the ground beef and diced onion for 7 to 8 minutes until browned and softened.
Add the minced garlic and cook for 1 minute. Drain excess grease from the skillet.
Stir in the cream of mushroom soup, sour cream, salt, and black pepper until the mixture is smooth and creamy.
Grease a 9x13-inch baking dish and spread the beef mixture evenly across the bottom.
Sprinkle 1½ cups of cheddar cheese over the beef mixture.
Arrange the frozen tater tots in a single layer across the top of the casserole.
Bake uncovered for 35 to 40 minutes until the tater tots begin turning golden brown.
Sprinkle the remaining cheddar cheese over the casserole and return to the oven for 5 to 10 minutes until the cheese melts and the filling bubbles.
Allow the casserole to rest for 10 minutes before serving.
Notes
Keep the tater tots frozen until ready to use for maximum crispiness. Drain the beef thoroughly to prevent a watery casserole. For best results, arrange the tater tots in a single layer and reserve some cheese for the final few minutes of baking. Let the casserole rest for 10 minutes before serving. Freeze unbaked for up to 3 months or refrigerate leftovers for up to 4 days.