Combine ground beef, onion, mustard, Italian seasoning, cumin, pepper and salt, mixing lightly but thoroughly. divide beef mixture into 4 equal portions shape into 1 cm thick patties, 10 cm in diameter. Place patties on rack in broiler pan so surface of meat is 3 to 10 cm from heat. Broil 10 minutes or to desired doneness, turning once. Place 2 or 3 lettuce leaves on each of 4 plates; top with 2 tomato slices, 2 onion rings and 1 burger.