Preheat oven to 375°F. Spray 9x13-inch baking dish with non-stick spray.
Brown ground beef and onion in large skillet over medium-high heat, 6-8 minutes. Break meat into crumbles.
Drain excess fat. Add garlic powder, salt, pepper, ketchup, mustard, and pickle relish. Stir well.
In saucepan, melt butter over medium heat. Whisk in flour, cook 1 minute.
Gradually add milk while whisking. Cook until thickened, 3-4 minutes.
Remove from heat, stir in 2 cups cheddar cheese, salt, and pepper until smooth.
Spread beef mixture in prepared baking dish. Pour cheese sauce over beef.
Arrange frozen tater tots in single layer across top.
Bake 35-40 minutes until tater tots are golden and crispy.
Sprinkle remaining ½ cup cheese on top. Bake additional 5 minutes.
Let rest 10 minutes before serving.