1Prepare white or yellow sweet onion by slicing in thick slices, place in plastic bag or glass jar with ice water, seal and refrigerate until ready to use. (This makes the onions very crisp and juicy.)
2Slice a vine-ripened tomato into 4 thick slices.
3Mix sour cream, chopped green onions, bread crumbs, worcestershire sauce and lemon pepper seasoning.
4Mix this in with the ground meat until combined well.
5Shape into 4 patties.
6Cooking the hamburger patties: GRILL: Place meat on grill over hot coals and cook to desired doneness, turning only once or twice.
7BROIL: Place meat in broiler pan; broil to desired doneness.
8MICROWAVE: Place meat in a microwave-safe dish and cover loosely with plastic wrap (made for microwave purposes); vent one edge slightly and microwave on highest power for 3 minutes.
9Turn patties over and cook 3 minutes longer or until desired doneness and rotate the dish 1/4 turn every 1-2 minutes and drain well.
10Mix the mayonnaise, mustard and ketchup and blend well.
11To serve, divide the ketchup mixture evenly on the warmed hamburger buns, top with a meat patty, tomato slice, onion slice and shredded lettuce.