Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
Step 2: In a large skillet over medium-high heat, cook the ground beef until it’s fully browned. Break it up into small crumbles as it cooks. Drain off any excess grease thoroughly.
Step 3: Add the chopped onion to the skillet with the cooked ground beef and sauté until softened, about 5-7 minutes. Stir in the garlic powder, onion powder, salt, and black pepper. Cook for another minute until fragrant.
Step 4: In a large mixing bowl, combine the condensed cream of mushroom soup, milk, and sour cream (if using). Whisk until smooth.
Step 5: Add the cooked beef and onion mixture to the soup mixture. Stir in 1.5 cups of the shredded cheddar cheese. Mix everything well until fully combined.
Step 6: Spread the beef and cheese mixture evenly into the prepared baking dish.
Step 7: Arrange the frozen tater tots in a single layer over the beef mixture. Top with the remaining 1/2 cup of cheddar cheese and the 1 cup of mozzarella cheese (if using).
Step 8: Bake for 30-35 minutes, or until the tater tots are golden brown and crispy, and the cheese is bubbly and melted. Let the casserole rest for 5-10 minutes before serving.