Cut the head of garlic in half cross-wise, leaving the paper on the cloves.
In a stock pot, combine the chicken thighs, scallions, cilantro, halved garlic head, and salt and pepper to taste. Cover with 8 cups of water. Bring to a boil and then reduce heat, cover, and simmer for 1 hour. Skim any scuzz or foam from the top of the broth occasionally.
Carefully remove the chicken and the garlic halves from the broth and transfer to a bowl. Strain the broth through a fine mesh sieve into a large bowl and discard the solids. Set the broth aside for later. Wipe out the pot.
Once the chicken is cool enough to handle, remove the skin and set aside in a bowl. Pick the chicken meat from the thighs and discard the bones. Using two forks, shred the chicken meat. Using a paper towel, carefully pop the cloves of garlic from the skin and roughly chop the cloves. Reserve the chopped garlic for the soup.