Place the hominy in a large soup pot with cold water at least 3 inches on top. Take off the dried skin layers from the head of garlic and add it into the pot. Do not add salt, because the hominy will toughen. Bring to a boil, then gently simmer over low medium heat uncovered for 3 hours or until hominy is tender and has begun to "bloom" or open up. Alternatively, you can buy precooked hominy and continue from this point.
In the meantime, place chicken in a large soup pot and cover with at least 1 inch of water above. Add white onion, cilantro and a tablespoon of salt and bring to boil. Simmer uncovered until chicken is cooked and tender, about 30 minutes. Drain and reserve the cooking liquid. When the chicken is cool enough to handle, remove the skin and the bones, and shred the meat into bite size pieces.
Meanwhile, make the green pozole sauce. Place tomatillos, garlic and chile in a medium 3-quart saucepan. Cover with water and set over medium-high heat. Bring to a simmer; cook until the tomatillos have changed color from a bright to a dull green and are soft but not breaking apart, about 10 minutes. Remove from the heat and reserve 1/2 cup of the cooking liquid. Drain the cooked vegetables and set aside.
In a blender, add toasted pumpkin seeds and chop until finely ground. Then add the cooked tomatillos, jalapeños and garlic, onion, salt and reserved liquid. Puree until smooth. Heat the oil in a saucepan over medium high heat until hot. Add the tomatillo sauce from the blender. Bring to a boil and simmer 15 to 18 minutes, stirring occasionally, so it will thicken, season and deepen its color.
When the hominy is ready, incorporate the shredded chicken and its cooking broth. Add the green pozole sauce and the epazote or cilantro. Let it cook for 30 minutes more. Check for seasoning - at this point I always add more salt - and serve.
You may present the Pozole in a big soup pot and place the garnishes in smaller bowls on the side. Each person can serve Pozole in their individual soup bowls, and then add as many garnishes to their soup as they would like. I do, however, recommend that some fresh lime juice be squeezed into it! Tostadas or totopos are eaten on the side.