In a small saucepan cook onion and garlic in melted margarine or butter until tender. Stir in the chili powder, cumin, and ground red pepper. Cook for 1 minute. Brush half of the onion mixture over chicken.
Grill chicken on an uncovered grill directly over medium coals for 6 minutes. Turn chicken; brush with remaining onion mixture. Grill for 6 to 8 minutes more or until chicken is tender and no longer pink. (Or, place chicken on the unheated rack of a broiler pan. Broil 5 to 6 inches from the heat for 10 to 12 minutes, turning once and brushing with remaining onion mixture.) Serve with shredded lettuce, chopped tomato, and avocado slices, if desired. Makes 6 servings.