Preheat your oven to 375°F (190°C). In a large skillet, brown the ground beef over medium-high heat, breaking it apart with a spoon. Drain any excess grease. Add the chopped onion to the skillet with the beef and cook until the onion is softened, about 5-7 minutes.
In a medium bowl, whisk together the condensed cream of mushroom soup, milk, ketchup, mustard, garlic powder, salt, and black pepper until smooth. This forms the creamy, flavorful base for your casserole.
Spread the cooked ground beef and onion mixture evenly in a 9x13 inch baking dish. Sprinkle 1 cup of the shredded cheddar cheese over the beef. Pour the prepared creamy sauce evenly over the cheese and beef mixture. Arrange the frozen tater tots in a single layer over the sauce, covering the entire surface of the casserole.
Bake for 30 minutes, or until the tater tots are golden brown and crispy. Remove the casserole from the oven, sprinkle the remaining 1 cup of shredded cheddar cheese over the tater tots, and return to the oven for another 5-10 minutes, or until the cheese is melted and bubbly.
Let the casserole stand for a few minutes before serving. Garnish with your favorite cheeseburger toppings like chopped pickles, diced tomatoes, or shredded lettuce, and a drizzle of extra ketchup or mustard if desired. Serve hot and enjoy!