Preheat oven to 180°. Butter a 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 and relish. Add salt and pepper to taste. 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 salt and pepper. Remove the pan from heat and whisk in 1 1/2 cups of cheese until it is completely melted and the sauce is smooth.

Evenly pour the sauce over the meat mixture. Arrange potatoes in an even layer over the cheese sauce. You will extra tots.Bake in preheated oven for 35 minutes. Remove from oven and sprinkle remaining cheese onto the tots. Return to the oven for an additional 5 minutes, or until the cheese is melted.

If desired, sprinkle with fresh Italian parsley just before serving.