In a small saucepan, cook onion and garlic in olive oil over medium heat until tender. Stir in the chili powder, cumin, and paprika. Cook for one minute.

Rinse chicken; pat dry with paper towels. Brush half the onion mixture over chicken.

Preheat BBQ to medium heat. Grill chicken, uncovered, directly over medium heat, for six minutes. Turn chicken over and brush with remaining onion mixture. Grill for six to eight minutes more or until chicken is tender and no longer pink. (If you prefer to broil the chicken, place chicken on the rack of a broiler pan. Broil chicken five to six 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.