Preheat oven to 375°F (190°C).
In a large skillet over medium heat, cook the ground beef, diced onion, and minced garlic until the beef is browned and the onion is translucent, about 7-8 minutes. Drain any excess fat.
In a medium bowl, combine the condensed cream of mushroom soup, sour cream, milk, Worcestershire sauce, salt, and black pepper. Mix well.
Add the soup mixture to the cooked ground beef mixture and stir until well combined.
Spread the beef and sauce mixture evenly in a 9×13-inch baking dish.
Arrange the frozen tater tots in a single layer on top of the beef mixture.
Sprinkle the shredded cheddar and mozzarella cheeses evenly over the tater tots.
Cover the casserole tightly with aluminum foil.
To freeze: Place the covered casserole in the freezer for up to 3 months.
To bake from frozen: Preheat oven to 375°F (190°C). Remove the foil and bake the casserole covered with fresh foil for 45 minutes. Remove the foil and bake an additional 15 minutes until the tater tots are golden and cheese is melted and bubbly.
To bake fresh (not frozen): Bake covered with foil for 30 minutes, then remove foil and bake an additional 15 minutes until tater tots are golden and cheese is melted.