Whisk together all marinade ingredients in a small bowl. Place chicken in a large ziploc bag and pound to an even thickness. Pour the marinade over the chicken and seal the bag. Place in the refrigerator for 2-3 hours.
After chicken has marinated, broil (or grill) the chicken for approximately 5 minutes per side. Remove from the pan and tent with foil on a plate to keep warm. While the chicken is cooking, prep the vegetables.
In a large non-stick skillet, heat the oil over medium heat and then add the onions, bell peppers and mushrooms. Cook until crisp tender, tossing gently with tongs periodically. Add the remaining ingredients and cook another 2-3 minutes, stirring frequently.
Slice chicken thinly and toss with the skillet ingredients. Serve immediately with flour tortillas and top with lettuce, chopped tomatoes, sour cream, guacamole, shredded cheese, or whatever else you like best.