1. 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.
2. 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.
3. 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.
4. 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.
5. Transfer burgers to lettuce-topped buns and place a plum tomato slice on each. Serve warm. Makes 12 mini burgers.