Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish.
Heat a large skillet over medium-high heat. Add the ground beef and diced onion. Cook for 6–8 minutes, breaking up the beef, until browned and the onion is softened. Drain excess grease.
Reduce the heat to low. Stir in the cream of mushroom soup, sour cream, milk, garlic powder, salt, and black pepper until the mixture is smooth and well combined.
Spread the beef mixture evenly in the prepared baking dish.
Sprinkle the shredded Cheddar evenly over the beef mixture.
Arrange the frozen tater tots in a single layer over the cheese.
Bake uncovered for 40–45 minutes, or until the filling is hot and bubbling and the tater tots are golden and crisp.
Let the casserole rest for 5–10 minutes before serving.