Cook onion and garlic in oil until onions are translucent.
Add in chicken broth.
In a bowl combine 1 cup water with 1/2 cup masa harina. Mix until well blended and smooth.
Add into chicken broth and stir to combine.
Add in remaining 1 cup water, enchilada sauce, green chilies, corn and tomatoes into chicken broth mixture.
Add in salt, chili powder and cumin.
Bring to a boil.
Add in chicken and reduce heat and allow to simmer for 30-40 minutes, until thickened.
If it thickens too much add some additional water or chicken broth.
Notes
Slow Cooker Method:Cook and shred chicken and set aside. Saute onions and garlic in olive oil in a pan. Add all the ingredients except for the chicken into the slow cooker. Cook on LOW for 6 to 8 hours or HIGH for 4 hours. Add the cooked shredded chicken 30 minutes before serving to allow it to heat through.Instant Pot Method:Press saute on the Instant Pot. Add onion and garlic and sauté until soft, 3-4 minutes. Add sauteed onions and garlic and all ingredients except the chicken to the Instant Pot.. Stir to combine. Add the chicken breasts (uncooked and raw). Cover and cook on high pressure for 20 minutes. Do a quick or natural release. Remove chicken and shred. Add chicken back in and salt and pepper to taste.