Mix the marinade ingredients together in a blender or food processor until they are smooth. Put the chicken in a zip-lock bag or dish and pour the marinade over. Put in the refrigerator and marinate 1-2 hours.
Pull the chicken out of the refrigerator when you're ready to start slicing up the peppers and chopping the extra garlic, if you're using it.
Heat 1 Tablespoon of the oil in a skillet and cook the chicken until done, about 5 minutes per side, covered.
Remove the chicken to a plate and over with foil and let rest while you cook the peppers.
Heat another tablespoon of oil in the same skillet and cook the peppers, onions and garlic until soft, about 10 minutes, stirring periodically. Salt to taste.
Wipe the skillet out and use it to heat up your flour tortillas.
Slice the chicken and serve with the peppers and onions with the condiments at the table, letting everyone build their own fajitas.