1. Bring 8 cups water, bay leaf, half of onion, half of garlic, and salt to a boil, covered, in a large, heavy-bottomed pot. Reduce heat and simmer 10 minutes.
2. Add chicken and poach at a simmer, uncovered, until cooked through (20 minutes). Transfer chicken to a cutting board. Strain cooking liquid. Discard solids and reserve broth. When chicken is cool enough to handle, shred it coarsely.
3. While chicken cools, toast pumpkin seeds in a small skillet over low heat, stirring occasionally, until puffed (6-7 minutes). Grind to a coarse powder using either a spice grinder or a mortar and pestle.
4. Simmer tomatillos and remaining onion in remaining 1 cup water in a medium saucepan, covered, until tender (10-12 minutes). Drain liquid and puree vegetables in a blender with jalapeños, 1/4 cup cilantro, oregano, and remaining garlic.
5. Heat oil in a large, heavy-bottomed pot over medium heat. Carefully add puree and cook, stirring frequently, until thickened (10-12 minutes).
6. Stir in pumpkin seeds and 1 cup reserved broth. Simmer 5 minutes. Stir in shredded chicken, hominy, salt, and 3 more cups reserved broth. Simmer, partially covered, 20 minutes.
7. Stir in remaining 1/2 cup cilantro to serve.