In a large bowl, or a large zip-top bag (for easier clean-up,) add all the marinade ingredients and mix well to combine.
Slice the chicken breasts in about 1" strips and add to the bag with the marinade. Add the sliced bell peppers and onions and gently mix everything together to make sure all the ingredients are combined and coated. Marinate covered, or with the bag closed, in the refrigerator for 3-4 hours. (You can bake right away but the flavor is much better if everything has some time to sit.)
Preheat the oven to 400F/200C. Line a large, rimmed sheet pan with a little olive oil or a sheet of foil.
Spread the chicken and vegetables on the sheet pan in an even layer and place in the hot oven. Bake for about 15-20 minutes until the meat is cooked through, then place the pan under the broiler for about 5-10 minutes so everything gets a slight char.
Serve with warm tortillas, fresh cilantro, avocado, hot sauce and whatever else you desire. Enjoy!
*If substituting shrimp for the chicken, marinate for no more than 2 hours or the acid can start to cook the shrimp.
*To make this vegetarian, skip the chicken and add a few more veggies of choice.