Rinse skin-on chicken pieces under cold running water. Trim off extra pieces of fat. Place chicken in a large pot and add chicken stock, garlic, and salt. Bring to a boil, reduce heat, cover, and simmer until chicken is cooked through, about 25 minutes. Skim off any foam that rises to the surface.

Combine tomatillos, avocados, cilantro, water, lettuce, poblano chiles, and serrano peppers in a blender; blend until smooth. Strain and pour into the pot with the chicken. Bring to a boil and simmer for 10 minutes. Season with salt.

Stir in corn kernels and bring to a boil again. Simmer until flavors are well combined and corn is cooked through, about 10 minutes. Discard garlic and serve.