Heat the olive oil in a large cast iron skillet over medium-high heat.
Season the chicken thighs with salt, black pepper, cumin, smoked paprika, oregano, and cayenne pepper (if using).
Add the seasoned chicken to the skillet and sear for about 5-7 minutes on each side until browned. Remove the chicken from the skillet and set aside.
In the same skillet, add the diced onion and cook for 3-4 minutes until translucent.
Stir in the minced garlic, red bell pepper, and green bell pepper. Cook for another 5 minutes until the peppers are tender.
Add the diced tomatoes (with their juice) and lime juice to the skillet. Stir to combine.
Return the chicken to the skillet, nestling it into the vegetable mixture. Cover and simmer for 15-20 minutes, or until the chicken is cooked through (internal temperature of 165°F).
Remove from heat and garnish with fresh cilantro before serving.