Preheat the oven to 425°F (220°C).
Pat the chicken thighs dry with paper towels. In a small bowl, mix together the kosher salt, black pepper, smoked paprika, and garlic powder.
Rub the spice mixture evenly over both sides of each chicken thigh.
In a large bowl, combine the sliced red, yellow, and green bell peppers, sliced onion, minced garlic, dried oregano, and crushed red pepper flakes if using. Drizzle with 1 tablespoon of olive oil and toss to coat evenly.
Heat 1 tablespoon of olive oil in a large oven-safe skillet or cast-iron pan over medium-high heat.
Place the chicken thighs skin-side down in the skillet and sear for 4-5 minutes until the skin is golden brown and crispy. Flip the chicken and sear the other side for 2 minutes.
Remove the chicken thighs from the skillet and set aside.
Add the pepper and onion mixture to the skillet and sauté for 3-4 minutes until slightly softened.
Nestle the seared chicken thighs on top of the peppers and onions in the skillet, skin-side up.
Transfer the skillet to the preheated oven and bake for 25-30 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and the vegetables are tender.
Remove from the oven and let rest for 5 minutes.
Garnish with chopped fresh parsley before serving.