Preheat the oven to 375°F. In a large bowl, combine the ground beef, ½ cup breadcrumbs, milk, half the cheddar cheese, diced pickles, 2 tablespoons ketchup, mustard, salt, and pepper. Mix until evenly blended but do not overwork the meat.

Shape the mixture into a loaf and place it in a baking dish. Spread the remaining 2 tablespoons ketchup over the top. Bake for 45 minutes.

While the meatloaf bakes, prepare the onion rings. Place the flour in one shallow dish and the remaining ¼ cup breadcrumbs in another. Dip each onion ring first in the milk, then coat in flour, dip back in milk, and finish with breadcrumbs. Heat oil in a skillet over medium heat. Fry the coated rings in batches until golden and crisp, then drain on paper towels.

Remove the meatloaf from the oven and sprinkle the remaining cheddar cheese on top. Return it to the oven for 5 minutes until the cheese melts. Let the loaf rest for 5 minutes, then top with the fried onion rings before slicing.