Step 1Wash the chicken pieces and place in a large pot. Add enough water to cover and add the peeled half onion, garlic and 1 tablespoon salt. Bring to a boil, then reduce heat and simmer until the chicken is tender, about 45 minutes. Remove the chicken from the broth and set aside. Strain the broth and reserve.
Step 2In the meantime, wrap the unpeeled onion in foil and roast it in a 400-degree oven until soft, about 50 minutes. Unwrap and peel the onion and set it aside.
Step 3Roast the whole tomatoes in a large dry skillet until softened, about 10 minutes. Place the tomatoes in a bowl. Roast the garlic quickly in the same skillet for about 1 to 2 minutes and add to the tomatoes.
Step 4Wipe out the skillet and roast, separately, the almonds, raisins and sesame seeds; add to the tomatoes. Next, roast together the peppercorns, cloves, oregano and cinnamon until fragrant and add to the tomatoes.
Step 5Remove the stems, veins and seeds from the chiles. Roast the chiles in the same dry skillet until fragrant and softened but not dark, about 2 to 3 minutes.
Step 6Working in batches, spoon the tomato mixture from the bowl into the jar of a blender with the roasted onion and the chiles. Blend together with some of the reserved chicken stock until smooth and fluid. Strain.
Step 7Heat the oil in a heavy saucepan over medium-low heat. Add the blended mixture and the chopped chocolate and stir until dissolved, about 10 minutes. The texture should be fluid. Add bread crumbs to thicken if the sauce is too thin. If it's too thick, add more broth. Stir in the sugar and salt to taste and cook another 10 minutes. The flavor should be distinctly sweet.
Step 8To serve, place a portion of chicken on a plate and pour the mole sauce over.