Preheat oven to 375°F (190°C).
In a large skillet over medium heat, cook the ground beef until browned, breaking it apart with a spatula, about 5-7 minutes.
Add the diced onion and minced garlic to the skillet with the beef and cook until the onion is translucent, about 3-4 minutes.
Drain any excess grease from the skillet.
In a large mixing bowl, combine the cooked ground beef mixture, cream of mushroom soup, sour cream, shredded cheddar cheese, milk, Worcestershire sauce, salt, black pepper, thawed mixed vegetables, and stir until well combined.
Transfer the beef mixture to a 9×13-inch casserole dish and spread evenly.
Arrange the frozen tater tots in a single layer over the beef mixture, covering the entire surface.
Cover the casserole dish tightly with aluminum foil.
To freeze: Place the covered casserole dish in the freezer for up to 3 months.
To bake from frozen: Preheat oven to 375°F (190°C). Remove foil and bake casserole for 60-70 minutes or until the tater tots are golden brown and the casserole is heated through.
To bake from thawed: Thaw casserole overnight in the refrigerator. Preheat oven to 375°F (190°C). Remove foil and bake for 35-40 minutes or until tater tots are golden and casserole is bubbly.
Let the casserole rest for 5 minutes before serving.