Heat the oven to 400°F. Lightly grease a 9x13-inch baking dish.
Warm the oil in a large skillet over medium heat. Cook the onion until softened, about 3 minutes.
Add the ground beef and brown it, breaking it up, 5–7 minutes. Drain excess fat.
Stir in the garlic and cook 30 seconds. Mix in Worcestershire, salt, and pepper.
Fold in the condensed soup, sour cream, and mixed vegetables. Simmer 2 minutes to warm through.
Spread the beef mixture in the baking dish. Sprinkle half of the cheddar over the top.
Arrange tater tots in an even layer. Bake 30–35 minutes, until bubbling and the tots are crisp.
Scatter the remaining cheese over the casserole. Bake 5–7 minutes more to melt. Rest 5 minutes before serving.