Begin by heating a large skillet over medium heat. Add the ground beef and cook it for about 5-7 minutes, breaking it apart with a spatula until it turns a nice brown color. Once browned, toss in the diced onion and sauté for an additional 3-4 minutes until the onion becomes translucent. Season the mixture with salt, pepper, and garlic powder, then drain off any excess grease.

Reduce the heat to low and stir in the cream of mushroom soup along with the frozen mixed vegetables and sour cream (if using). Mix thoroughly until everything is well combined. For a smoother texture, add a splash of milk or chicken broth to the mixture. Allow it to cook for about 2 minutes, then remove from heat and let cool for around 10 minutes to prevent soggy tater tots.

Once your mixture has cooled, grease a 9x13 baking dish with cooking spray or a little butter. Spread the meat and vegetable mixture evenly across the bottom of the dish. Sprinkle your shredded cheese generously on top of the meat layer, ensuring it’s well-distributed. Next, take the frozen tater tots and arrange them in a single layer over the entire casserole, making sure to cover every inch.

Preheat your oven to 375°F (190°C) and place the assembled Tater Tot Casserole in the oven. Bake for 30–35 minutes, or until the tater tots are golden brown and crispy on top. Once it's done, remove it from the oven and let it rest for about 5 minutes before slicing and serving.