Preheat the oven to 375 degrees. Spray a casserole dish with nonstick cooking spray.
Add the ground beef and onion to a large skillet over medium heat. Break the beef into chunks. Cook undisturbed until the beef is browned on the bottom, then stir and continue to cook until beef is browned and onions are softened. Once browned, sprinkle with ½ teaspoon salt, remove from heat, and drain the fat.
Pour the cooked meat and onions into the prepared casserole dish. Add the cheese soup, pickle relish, ketchup, mustard, and 1 cup of the shredded cheese. Stir until well mixed.
Top with rows of tater tots. Sprinkle remaining salt on top.
Bake in the preheated oven for 35 minutes or until the tots are golden brown. Remove and top with the remaining cheese, then bake for 5 minutes more to melt. Serve immediately.