In a small pot or metal skillet, combine the cinnamon stick, sesame seeds, pepitas, guaque (guajillo) pepper, and pasilla or poblano peppers.
Toast the mixture over medium heat until fragrant, then let it cool.
Once cooled, blend the toasted mixture with 8 oz of water in a blender. Strain the blended mixture to remove any solids.
Preheat your oven to 350°F (175°C). Roast the tomatoes, tomatillos, red bell peppers, garlic, and onion in the oven for 15 minutes.
Allow the roasted vegetables to cool, then blend them with a bunch of cilantro to taste. Strain the sauce for a smoother texture.
Cut the chicken breasts into pieces and cook them in a large pot with approximately 12 cups of water and salt to taste.
Once the chicken is fully cooked, remove it from the pot and set it aside. Reserve the broth for thinning the sauce, if needed.
In a separate pot, combine the cooked chicken, carrots, güisquil (chayote), potatoes, and green beans. Cook for 2 minutes without adding liquid.
Add the previously blended sauces and the remaining cilantro to the pot.
Continue cooking over low to medium heat until all the vegetables are tender. If the sauce is too thick, thin it out with the reserved chicken broth.
Toast the corn tortillas on a grill or stovetop. Heat each side for 30 seconds, or until slightly crispy.
Serve the Pepián de Pollo in bowls, optionally accompanied by white rice and toasted corn tortillas. Garnish with sliced avocado if desired.