In a large skillet, heat over medium heat and add the ground beef along with diced onion. Cook until the beef is browned and the onion becomes soft and translucent, about 5-7 minutes. Season generously with salt, pepper, and garlic powder, then drain any excess grease from the skillet.

Stir in the cream of mushroom soup and frozen mixed vegetables into the skillet with the beef mixture. Combine thoroughly until everything is well-coated in the creamy sauce. If using sour cream, add it now, too. Thin the mixture with a splash of milk or broth, stirring until smooth.

Remove the skillet from the heat and set it aside for about 10 minutes to cool.

Grease a 9x13 baking dish with cooking spray or butter. Spread the beef and creamy vegetable mixture evenly across the bottom of the dish. Sprinkle a generous layer of shredded cheese over the top followed by arranging the frozen tater tots in a single, even layer atop the cheese.

Preheat your oven to 375°F (190°C) and bake the assembled Tater Tot Casserole for 30-35 minutes until the tater tots are golden brown and crispy. Let it rest for about 5 minutes before serving.