Preheat oven to 350?F. Butter a 2-quart casserole dish, or spray with nonstick cooking spray.

In a large skillet over medium heat, cook beef and onion, stirring occasionally, until beef is brown; drain.

Stir in ketchup, mustard, relish, and ? teaspoon salt. Taste and adjust seasoning as needed. Spoon mixture into prepared casserole dish and smooth into an even layer.

In a medium saucepan over medium heat, melt butter completely. Slowly whisk in the flour until smooth. Continue to whisk while cooking the roux until it is golden brown.

Slowly whisk in the milk, taking care to fully incorporate it with the roux. Keep whisking until the sauce is thick and bubbling; about 5 minutes. Stir in ? teaspoon salt and pepper.

Remove the pan from heat and whisk in 1 ? cups of cheese until it is completely melted and the sauce is smooth.

Evenly pour the sauce over the meat mixture.

Arrange the frozen tater tots in an even layer over the cheese sauce.

Bake in preheated oven for 35 minutes. Remove from oven and sprinkle remaining cheese over the top. Return to the oven for an additional 5 minutes, or until the cheese is melted.