Season ground beef with salt and pepper and form into 1-inch meatballs. Thread skewers with white onion squares, meatballs, pickle slices and large cherry tomatoes; brush with vegetable oil. Grill over a medium-high, foil-lined grill or grill pan, turning occasionally, until meatballs are cooked to desired doneness, about 8 minutes for medium. During last minute of grilling, brush generously with BBQ sauce, then place small squares of American cheese on top of each meatball. Cover until cheese melts.