1. Heat gas grill to medium-high or prepare charcoal grill with medium-hot coals. In a large bowl, mix together the ground beef, dried onion, mustard, 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. Transfer hamburger patties to the grill and cook 4 minutes. Flip and cook 4 additional minutes. 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 minute.
4. Slice buns in half horizontally and transfer burgers to buns. Place a plum tomato slice on each. Serve warm.