Bring a pot of water to a boil, add chicken, onion, and a pinch of salt. Cook until the chicken is fully cooked.
2
Prepare the vegetables: trim and slice the green beans; peel and cut carrots in half then into pieces; peel and cube the chayote; cut potatoes into chunks; and cut squash into pieces.
3
Once the chicken reaches a boil, add the firmer vegetables—green beans and carrots—so they cook together with the chicken.
4
Add the squash, chayote, and potatoes; season with salt and continue cooking.
5
In a separate pot, cook the guajillo chiles, tomatoes, onion, and garlic until softened.
6
Blend or grind the cooked chiles and tomatoes along with the garlic and onion into a smooth sauce.
7
Pour the chile sauce into the chicken broth, add epazote leaves if desired, and simmer for a few minutes to allow the flavors to blend.
8
Serve in bowls, optionally topped with lime juice and serrano chiles for extra flavor.