Begin by boiling the chicken thighs and drumsticks in the 18 cups of water with a big pinch of salt and the cilantro for about 20 minutes.
In a pan over medium heat, dry roast the tomatoes, bell pepper, tomatillos, onion, and garlic until cooked through and charred on all sides. Be careful not to burn any of the ingredients as this may make them bitter.
In a separate pan over medium heat, dry roast the guajillo and ancho chilies until lightly toasted but not burned, then set them aside.
In the same pan, dry roast the pepita and sesame seeds until just lightly toasted as well.
Transfer the roasted vegetables, chilies, seeds, and the cooked cilantro from the chicken broth to a blender. Add a burnt tortilla and 2 cups of the chicken broth to the blender as well and blend until it becomes a completely smooth sauce.
Add the blended sauce to the pot of broth and chicken, then allow this to simmer uncovered for 20 minutes.
Next, add the carrots, potatoes, and chayote, and simmer for another 15 minutes. Lastly, add the green beans and cook for a final 10 minutes or until all the vegetables are perfectly tender.
Add salt and pepper to taste, then serve with rice and tortillas. Buen provecho!