Preheat grill over medium high heat. Make the tzatziki sauce (you can also do this ahead of time and store it in the fridge).
Combine all burger ingredients in a large bowl, and use hands to combine. Form into 4 patties approximately 1 inch thick.
Grill the burgers for 3-4 mins each side for medium/medium well, and 5 mins each side for well done. Serve the burgers on the lettuce "buns" with the sliced tomato, red onion, and tzatziki sauce. Enjoy!