Preheat oven to 400°F. In a large cast iron skillet (or other oven-proof skillet), brown ground beef and onion over medium-high heat until the meat is no longer pink, about 5-7 minutes. Add the garlic and cook for 1 more minute. Drain off excess grease.
Stir in the diced tomatoes, tomato sauce, ketchup, mustard, and Worcestershire sauce. Bring to a boil, and then reduce the heat so that the mixture is gently simmering. Simmer uncovered until the sauce thickens slightly, stirring occasionally, about 5-7 minutes. Remove from the heat. Stir in chopped pickles (if using) and 1 cup of the cheddar cheese. Taste and season with salt and pepper, if desired.
Arrange the tater tots in a single layer on top of the beef mixture. Transfer to the oven and bake until the tater tots are golden brown and the filling is hot and bubbly, about 30 minutes. Sprinkle the remaining ½ cup of cheese on top and continue baking until the cheese melts, about 5-10 more minutes. Garnish with chopped fresh parsley or chives.