Place chicken in a plastic bag set in a shallow baking dish.
In a small bowl, stir together olive oil, the 1/4 cup cilantro, lemon juice, 1 tablespoon garlic, and salt. Pour over chicken in plastic bag. Close bag. Marinate in the refrigerator for 2 to 4 hours, turning bag occasionally.
Drain chicken, reserving the marinade. Place chicken on the rack of an uncovered grill directly over medium coals. Grill for 12 to 15 minutes or until tender and no longer pink, turning once and brushing once with reserved marinade halfway through the grilling. Discard remaining marinade.
Meanwhile, in a blender container or food processor bowl, combine tomatillos, onion, 1 clove garlic, jalapeno pepper, and the 1 tablespoon cilantro. Cover and blend or process until sauce is nearly smooth.
Just before serving, pour sauce into a saucepan. Bring to boiling; reduce heat. Simmer, uncovered, for 5 minutes or until the desired consistency.
Remove chicken from grill. Slice across the grain into 1/2-inch-thick slices. Spoon warm sauce over chicken. Serve immediately. Makes 6 servings.
When working with chili peppers, wear plastic or rubber gloves because chili peppers contain volatile oils that can burn your skin and eyes. If your bare hands touch the chili peppers, wash your hands well with soap and water.