In a small bowl, combine the onion and tomato slices with the lemon juice and 3 tablespoons of the olive oil. Turn to coat evenly and set aside.

Prepare a fire in a charcoal grill or preheat an indoor grill or broiler. In a large bowl, combine the meat, salt, pepper, chili sauce and ice water. (The ice water helps to keep the burgers juicy.) Using a fork, stir together until well mixed. Then, using your hands, form the meat mixture into 4 patties each 1 inch (2.5 cm) thick, being careful not to handle the meat too much. Set aside.

Place the onion rolls or hamburger buns, cut side down, on the grill rack or a broiler pan; grill or broil until just golden, 2--3 minutes. Transfer to individual plates, cut side up, and brush the remaining 1 tablespoon olive oil lightly over the toasted buns.

Position the grill or broiler rack 4 inches (10 cm) from the heat source, then grill or broil the burgers, turning once, for 6 minutes on the first side and 4-5 minutes on the second side for medium-rare to medium.

Place each burger on the bottom half of the toasted bun, top with a slice each of red onion and tomato, and sprinkle with a little of the lettuce. Cover with the top bun and serve immediately.