For the salsa: In a bowl, combine the tomato, garlic, onion, green peppers, jalapeno peppers, cilantro, and salt. Set aside 2 tablespoons of the salsa. Cover and chill the remaining salsa until serving time.
In another bowl, combine the ground beef and the 2 tablespoons of reserved salsa; mix well.
Shape the mixture into six 1/2-inch-thick oval patties.
Grill the patties on an uncovered grill directly over medium coals for about 13-15 minutes (or until an instant-read thermometer inserted in side of patty registers 160 degrees F), turning the patties once halfway through grilling time.
Arrange the shredded lettuce on individual plates. Top with the burgers, the remaining salsa, and the cheddar cheese.
Serve with sour cream and/or guacamole.