1 4- to 5-poundchicken, cut up, breasts split and cut in half (10 pieces)
Heat a comal or small skillet over medium heat, and toast the stemmed, seeded ancho chiles until the skin changes color and the chiles begin to release fumes, about 1 minute. Remove from the heat and add the onion and garlic clove to the comal or skillet. Toast, flipping the onion and garlic clove from side to side, until charred on the outside and softened inside, 10 to 12 minutes. Remove from the heat, set aside and when cool enough to handle, peel the garlic clove. Alternatively, char the onion and garlic under the broiler. Preheat the broiler with the rack arranged at the highest setting and cover a small sheet pan with foil. Broil the garlic for 5 to 8 minutes, turning halfway through, and the onion for 10 to 12 minutes, flipping halfway through.
Bring the chicken broth to a simmer in a medium saucepan. Add the pecans, prunes and toasted ancho chiles. Simmer for 10 to 12 minutes, until the chiles and prunes have rehydrated and plumped, and the pecans have softened. Set aside and let cool. Transfer to a blender, add the charred onion and garlic, and puree. If your blender is small, do this in batches.
Season the chicken with the salt and pepper.
Heat the oil in a large casserole or wide, heavy lidded skillet over medium high heat. Once hot, brown the chicken pieces, in batches, skin side down first then skin side up, until nicely colored, about 4 to 5 minutes on each side. Transfer to a bowl or plate as each batch is done.
Reduce heat to low, and using the lid of the casserole or pan as a shield for splatters, pour in the pureed pecan sauce. It should bubble and splutter dramatically. Stir well, scraping all the bits up from the bottom of the pan, and return the chicken pieces to the pan. Cover, turn down the heat to medium low, and simmer for 40 minutes, stirring and scraping the bottom of the pan from time to time, until the chicken is completely cooked through and the sauce is thick and delicious. Taste and adjust seasoning.