In a large skillet, brown the ground beef over medium heat until fully cooked, breaking it apart with a spatula as it cooks. Drain any excess fat.
Add the diced onion and minced garlic to the skillet with the beef, cooking until the onion is translucent, about 5 minutes.
Stir in the creamy mushroom soup, milk, Worcestershire sauce, mustard powder, salt, and pepper. Mix until everything is well combined and heated through.
In a large greased casserole dish, layer the beef mixture evenly on the bottom.
Sprinkle the cheddar and mozzarella cheese over the beef mixture, spreading it evenly.
Top with the frozen tater tots, arranging them in a single layer over the cheese, covering the entire surface.
Bake in the preheated oven for about 30-35 minutes or until the tater tots are golden brown and crispy.
Remove from the oven and let it cool slightly before serving.
Garnish with chopped green onions to add a fresh touch.
Notes
Let the casserole cool slightly before serving for better slicing.