To make the marinade: Use a shallow, non metal container and combine the ingredients. Rinse the chicken and pat dry with paper towel. Add the chicken to the marinade, turning to coat each side. Cover and refrigerate for 4 hours, turning occasionally.
Remove the chicken breasts from the marinade and discard the marinade. Cook the chicken breasts over direct/medium heat. Preheat a hotplate over direct/high heat and cook the onion slices, tomato slices and capsicum
quarters. The chicken and onion will take 4 to 8 minutes each side and the tomato and capsicum will take 3 to 4 minutes each side.
Wrap the tortillas in foil and heat for about 1 minute. Cut the capsicum into strips and slice the chicken. Place the chicken, onion, tomatoes, capsicum and avocado in warm tortillas and roll up to eat. Serve with salsa.