1. Heat gas grill to medium-high or prepare charcoal grill with medium-hot coals. In a large bowl, mix together ground beef, onion, Worcestershire sauce, garlic salt, Italian seasoning and black pepper until well combined.
2. Shape seasoned beef into 6 patties.
3. Split rolls in half and toast cut-side down on grill, about 1 minute. Transfer to a large platter. Top buns with arugula leaves.
4. Transfer hamburger patties to grill and cook 2 minutes. Flip and cook 2 additional minutes or until cooked through. Top burgers with cheese. Grill until slightly melted, 1 to 2 minutes.
5. Transfer burgers to lettuce-topped buns and place a tomato slice on each. Serve warm.