Heat a thinly oiled cast iron skillet to medium high or high heat. Preheat oven to roast at 350 degrees.
Pat dry chicken thighs, leaving skin on. Season both sides of thighs with kosher salt and ground black peppercorn.
Once skillet is hot, place thighs in skillet, skin side down. Let skin sear for approximately 3-4 minutes, or until golden brown and crispy. Turn thighs over and cook for another 2 minutes.
Remove thighs from skillet onto a clean plate when finished, and set aside. Lower skillet heat to medium or medium low.
A minute later, add a tablespoon of olive oil, then throw on the sliced onions. Let onions cook for approximately 7 minutes. Stir occasionally. It's okay to have a bit of a char to them! Also, the sweat from the onions should bring up the scrapings from the chicken on the bottom of the skillet.
Once onions are softened and darkened a bit, add the sliced peppers. Be sure to stir around so the peppers can reach the bottom of the pan. Let cook for approximately 7-10 more minutes, stirring occasionally.
Up your heat to medium if you haven't already. Add garlic, water and gluten-free soy sauce and mix thoroughly. Let sautee for approximately 2 minutes.
Take the chicken thighs that were set aside, and place in skillet with vegetable mixture. Place thighs around skillet so they are laid out evenly, preferably not touching. And push the vegetables to the side a bit so they are not completely under chicken.
Sprinkle koshers salt and pepper over everything to taste, then place cast iron skillet in heated oven. Turn off stovetop range.
Cook in oven for approximately 40-45 minutes, or until chicken juices run clear and thighs reach an internal temperature of about 165 degrees.
You can baste the chicken with the sauce if desired halfway through cooking time. If things look like they are drying out, add a small amount of water to the sauce.
Once cooked, remove from oven then plate and serve immediately. Offer cilantro, avocado, jalapeno peppers and sour cream on the side for anyone who wants it.