Heat gas grill to medium-high or prepare charcoal grill with medium-hot coals. In a large bowl, mix together the ground beef, onion flakes, Worcestershire sauce, garlic salt, Italian seasoning and black pepper. Mix until well combined.
Shape seasoned beef into 12 small patties, about 1/4-cup mixture for each, forming patties that are 2-1/2 inches across and 1/2 inch thick.
Split the rolls in half and toast the cut side on the grill, about 1 minute.Transfer to a large platter. Top buns with arugula leaves.
Transfer hamburger patties to the grill and cook 2 minutes. Flip and cook 2 additional minutes or until cooked through. Top burgers with cheese. Slice should be roughly the size of the burger; if not, cut or fold it to fit. Grill until slightly melted, 1 to 2 minutes.
Transfer burgers to lettuce-topped buns and place a plum tomato slice on each. Serve warm. Makes 12 mini burgers.