Combine marinade ingredients in bowl and whisk to combine. Add chicken thighs and marinate for a couple hours. For ease of clean up, use a ziplock bag!
Meanwhile, slice peppers into quarters and remove seeds. Slice the red onion into a few thick slices. Lightly brush veggies (including whole green onions, if desired) with a little olive oil and sprinkle with salt and pepper. Set aside until ready to grill.
Heat grill. Alternatively you can use a cast iron grill pan on the stove or other indoor type grill. Remove chicken from marinade and grill, along with the veggies, until everything is cooked. Slice chicken and veggies. Serve with warmed tortillas, top with cheese, cilantro, and avocado.
I have only made this on the outdoor bbq and indoors on my cast iron grill pan on the stove. If you have neither don’t let that stop you, just cook it like you would a steak!
If you like, add a squeeze of lime, pico de gallo or other toppings!