Step 1Place the chicken in a Dutch oven. Add 1 onion half, 1 clove of garlic and water just to cover. Simmer until the chicken is cooked, about 30 minutes. Drain and reserve the stock, adding salt to taste. Set the chicken aside and keep warm.
Step 2Place 4 cups of the chicken stock in a saucepan. Add the masa and heat over medium-low heat, whisking often to dissolve. Bring to a boil and simmer, whisking often, until thickened and the consistency of heavy cream, about 15 minutes.
Step 3Remove the husks from the tomatillos, then rinse and place in a small saucepan. Add water to cover and bring to a boil, cooking until the tomatillos are softened, 5 minutes. Drain.
Step 4In a blender container, combine the tomatillos, hierba santa, epazote, jalapeno, the remaining onion half, the remaining 5 cloves of garlic and the cumin and blend until finely ground. Add to the thickened broth, season with salt to taste and simmer gently until heated through, about 3 to 4 minutes. To preserve the green color, do not allow to boil.
Step 5Divide the chicken among serving plates and pour the sauce over the top.