In a small bowl, combine all of the Chicken Marinade ingredients; whisk together until fully combined. Use a fork to pierce each chicken breast in several places and then place the chicken breasts in a medium-sized glass baking dish. Add Chicken Marinade to the baking dish; cover and refrigerate overnight.
Place a large cast iron skillet over medium-high heat. Once skillet is hot, add olive oil and peppers. Sauté for 6-7 minutes, and then add the onions. Continue sautéing for 15-20 more minutes or until onions and peppers soft, stirring occasionally.
Meanwhile, place another skillet or fry pan over medium-high heat. Place marinated chicken breasts in pan and cook until done (~15-18 minutes), flipping after 10 minutes.
In a small bowl, whisk together all of the ingredients for the Vegetable Finishing Sauce. When the peppers and onions are almost done, reduce heat to medium low, add the Vegetable Finishing Sauce and simmer for 2-3 more minutes.
Once chicken breasts are cooked, transfer them to cutting board and slice thinly.
Place chicken, onions and peppers on a flour tortilla. Top with optional toppings as desired. (Tip: Wrap the stack of tortillas in a wet paper towel and microwave for 30 seconds before adding toppings. This will both warm and soften the tortillas.)