Heat oven and season chicken. Heat oven to 300°F. Pat chicken dry and season chicken pieces on all sides with salt and pepper.

Brown chicken. Heat oil in a large, oven-safe frying pan or Dutch oven over medium-high heat. Add chicken pieces and brown on all sides, about 8 minutes. You may have to do this in batches depending on the size of your pan. Transfer chicken to a large plate, or baking sheet.

Add vegetables and deglaze pan. Add onion, celery, carrots, and garlic to the pot and cook until vegetables are becoming soft and browned, season with salt and pepper. Add broth and bring to a boil, scraping up any browned bits stuck to the bottom of the pan.

Braise chicken. Return chicken to the pan and add oregano. Cover and place in the oven. Braise chicken until tender, about 1 hour.

Strain chicken and save chicken broth. Let cool, then remove chicken pieces to a large plate or baking sheet. Place a large fine mesh sieve over a large bowl and pour the remaining contents of the pan through the sieve. Discard the vegetables, but save the broth for the mole. This can be done a day in advance.

Start the mole sauce. Heat the 2 tablespoons of oil in a medium frying pan over medium heat. Add pumpkin seeds and toast until they become golden and start to pop, about 3 minutes. Remove to a large plate with a bamboo skimmer or slotted spoon, leaving any excess oil in the pan.

Toast almonds and sesame seeds. Add almonds to the pan and toast until fragrant, about 3 minutes. Remove to the plate with the pumpkin seeds. Add sesame seeds to the oil and toast until golden, about 2 minutes. Remove to the plate with the other seeds and nuts, leaving any excess oil in the pan.

Boil tomatillos and peppers. Combine tomatillos, jalapeños, and poblanos in a large pot. Cover with water and salt water generously. Bring to a boil and cook until tomatillos are soft, but not falling apart, about 7-10 minutes. Strain vegetables, saving 2-3 cups of the cooking liquid.

Blend the mole in batches. To a blender, add a spoonful of the tomatillo mixture, 1 cup of the reserved chicken broth, a spoonful of the nut mixture, a few onions, a garlic clove or two, some cilantro, and some basil. This is going to depend on the size of your blender, only fill the blender about 2/3 of the way full so it has room to blend.

Blend on high until very smooth, about 2 minutes. Pour into a large bowl and keep blending more ingredients, adding all the chicken broth and then some of the tomatillo cooking liquid 1/4 cup at a time if you need more liquid to get the blender moving.

Add salt and coriander to the last batch. To the last batch in the blender, add 1 1/2 teaspoons salt and the coriander.

Fry mole sauce. Heat 1/4 cup oil in a large Dutch oven over medium heat. Once oil is shimmering, add mole sauce (be careful, sauce will spatter) and fry the sauce, stirring constantly for the first few minutes. Bring to a boil, then reduce to a simmer. Cook, until sauce is thickened slightly. Taste and add more salt if needed.

Warm chicken in the sauce. Add chicken and let simmer until chicken is warmed through. You can serve right away, although it is best to make this a day in advance and let the flavors meld overnight before serving.