Preheat your oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.
In a large skillet over medium-high heat, brown the ground beef with diced onions and garlic until the meat is no longer pink (about 8-10 minutes).
Drain any excess fat from the meat mixture and return to the skillet.
Stir in the cream of mushroom soup, milk, Worcestershire sauce, and mustard powder. Season with salt and pepper.
Pour the meat mixture into your prepared baking dish.
Layer 1 cup of shredded cheese over the meat mixture.
Arrange the frozen tater tots in a single layer on top.
Bake for 45 minutes, then sprinkle the remaining cheese on top.
Return to the oven for an additional 5-10 minutes, until the cheese is melted and bubbly.