Heat a cast-iron griddle until very hot. Layer the pickle slices on the bottom buns.
Place 4 Rocket Meatballs on the griddle. Cook them over moderately high heat for 30 seconds. Using a sturdy large spatula, flatten each ball into a 5-inch round patty. Season the patties with salt and pepper and cook for 2 minutes, until well seared. Press a handful of sliced onions onto each patty.
Using the spatula, carefully flip each burger so the onions are on the bottom. Top with the cheese and cook for 2 minutes. Cover with a roasting pan and cook just until the cheese is melted, 1 minute more. Transfer the burgers with the onions to the buns.