Preheat oven to 400°F (200°C). Brown beef and onion in an oven-safe skillet over medium-high heat, 6 to 8 minutes. Drain fat, add garlic, and cook 30 seconds.

Stir in ketchup, mustard, Worcestershire, pickle relish, salt, and pepper. Simmer 2 minutes.

Remove from heat. Top with half the cheese, then arrange frozen tater tots in a single layer.

Bake 20 minutes. Sprinkle remaining cheese over tots, bake 5 more minutes until golden and bubbly. Let sit 5 minutes before serving.