1. In a large bowl, mix together the beef, lemon zest, salt, black pepper and garlic. Divide meat into four equal pieces and gently form into 1/2-inch-thick patties.
2. Heat the grill to medium high and brush the grate generously with oil. Transfer burgers to grill. Close the cover and cook 2 to 3 minutes a side for medium rare. Remove from grill and rest 5 minutes.
3. Arrange burgers on the buns and top with the tomato, onion and lettuce.