Place onion, peppers, celery, and garlic in a blender and pulse until smooth. Set aside.
Set oven to broil. Place tomatillo halves cut side down in a cast iron pan. Broil until skins are charred and tomatillos have released their juices, about 5 minutes.
Heat oil over medium high in a heavy bottom pot or Dutch oven.
Season chicken with salt. Place chicken parts in pot and brown, about two minutes per side. Work in batches to avoid crowding the pot.
Remove chicken from pot. Add in onion and pepper mixture to the pot. Reduce heat to medium and continue to cook, stirring often, until mixture becomes golden, about 30 minutes.
Place the chicken pieces back in the pot along with the tomatillos and all the juices that were released in the pan.
Pour chicken stock into pot and increase heat to high. Bring to an immediate boil then reduce heat to low and cover. Simmer until chicken is cooked through and tender, about 90 minutes.
Season with salt and pepper. Pour in hominy and stir to warm through.
