In a large bowl, combine the beef, seasoning mix, onion, and pepper and mix well. If desired, add bread crumbs to meat mixture, too.
With wet hands shape the meat mixture into 6 patties, each about 1-inch thick. Grill the burgers for 5 to 7 minutes per side or until an internal temperature of 165 degrees F is reached and juices run clear.
Serve the burgers topped with the lettuce and tomato.