Pre-heat a grill pan, outdoor grill or a large nonstick skillet over medium-high heat.
In a medium size mixing bowl, combine the meat with onion, ketchup and mustard. Form twelve large meatballs. Flatten the meatballs into small patties and season with salt and pepper. Drizzle the meatballs with EVOO to keep them from sticking to the cooking surface and place them on the pre-heated grill pan or grill. Cook for 3 minutes on each side, then add the cheese to melt over the patties. On an outdoor grill, close the lid to melt the cheese. In a skillet or on a grill pan, tent the mini cheeseburgers loosely with foil to melt the cheese.
Arrange the mini cheeseburgers on an individual lettuce cup and top with the chopped tomatoes and pickles.