Preheat the oven to 350 degrees F (175 degrees C).
Season chicken thighs with salt and pepper.
Heat oil in a cast iron skillet over medium heat. Cook chicken in the hot oil without moving to blacken skin, about 4 minutes. Turn chicken and cook about 2 more minutes. Transfer chicken to a plate.
Add butter to the skillet. Cook chile peppers and onion in the melted butter until crisp-tender, 5 to 6 minutes. Add garlic and cook until fragrant, about 1 minute. Stir in cream until combined. Season with taco seasoning, salt, and pepper.
Place chicken on top of onions and chiles and cover with foil.
Place in the preheated oven and cook until chicken is no longer pink at the bone and the juices run clear, 20 to 25 minutes. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C).