Preheat oven to 350°F. Spray a casserole dish with nonstick cooking spray.
Cook beef and onion in a large skillet over medium heat, occasionally stirring until beef is brown. Drain the excess fat.
Stir in mustard, ketchup, and relish. Add salt and pepper to taste. Pour mixture into a prepared casserole dish in an even layer.
Melt butter in a medium saucepan over medium heat.
Slowly add in the flour and whisk until smooth. Continue to whisk and cook until golden brown.
Arrange the frozen tater tots in an even layer over the roux (you will have extras).
Slowly add in the milk, whisking it to be fully incorporated with the roux. Keep whisking until the sauce is bubbly and thick (about 5 minutes). Add salt and pepper.
Remove from the heat and whisk in 1-1/2 cups of the cheddar cheese until it is fully melted and smooth.
Pour the roux evenly over the meat mixture.
Bake in the oven for 35 minutes. Remove from the oven and sprinkle the rest of the cheese onto the tots. Bake for an additional 5 minutes, or until the cheese is melted.
Garnish with parsley before serving if you'd like!