Open the can of hominy and rinse well in a colander until the water is clear. Place the hominy in your largest pot, considering the hominy and the whole chicken are going to need to fit. If you don’t have a large enough pot, split everything in half and place into two pots.
Clean and cut the chicken into 4 pieces and place it all in the pot with the hominy. Take the two garlic heads whole, rinse them and throw them in the pot, too. Lastly, put water in the pot until it covers everything so far. Bring it to boil and then lower the temperature to medium and let it simmer for 45-55 minutes or until the chicken is cooked.
Take the chicken out of the soup and let it cool.
Remove garlic heads from the broth. Then, you can do one of two things: Discard them or cut off the ends of the heads, squeeze the garlic pulp out of their skin, and return the pulp to the broth along with the salsa.
In the meantime, prepare the salsa: You may need to do this part in batches, unless you have an industrial blender. In a blender, mix well the 4 cloves of peeled garlic, half an onion, two pounds of tomatillos (peeled and wash), serrano chiles (to taste, one for flavor, two for heat), cilantro, salt, and pepitas. Add up to two cups of water in order to blend smoothly.
Add the salsa to the soup pot with the hominy and let it simmer for 15-20 minutes on low heat until the salsa changes in color from bright green to olive green. Taste for salt. The soup is ready to serve.
While the broth is simmering with the salsa, shred the cooked chicken by hand. At this point, you have two options, either add the shredded chicken straight to the pot or keep the chicken separate and add a little bit to each bowl as you serve it to make sure everyone who wants some gets some.