In a large pot, add 2 1/2-3 lbs chicken pieces, drumsticks, thighs, skin on bone in. Cover with 10-12 cups of water. Add 1 large onion cut in half, half head of garlic, 1 carrot, 1 stick of celery, 4 bay leaves, 1 tsp peppercorns, handful of fresh cilantro and salt to taste. Bring up to a light simmer. Skim off the top as needed. Cook for 40 minutes. Set aside.
Once broth is ready, in a large deep skillet, add 2 tablespoons of oil and heat to medium. Add the onion, garlic, jalapeño, poblano and tomatillos. Let them begin to caramelize a bit, then stir occasionally for the next 7-8 minutes. Transfer ingredients to the blender jar.
In that same skillet, add the pepitas, sesame seeds. oregano and cumin seeds. Drizzle in another 2-3 tablespoons of oil. Continue cooking at heat below medium until seeds begin to pop and toast. Stir often, so they don't burn. Cook time is about 5 minutes, more or less. Transfer ingredients to blender jar with tomatillo mixture.
To the blender jar, ladle in 4-5 cups of reserved broth. Carefully, cover and hold the lid down with a clean kitchen towel. Blend on high until very smooth. You may need to blend for a few minutes.
Once again, to that same deep skillet, preheat 2 tablespoons of oil at medium heat for a few minutes. Once hot, pour in the pipian sauce from the blender. Keep lid handy, as it can splatter and pop. Stir well to combine. Ladle in a few more cups of reserved chicken broth. Once combined, taste for salt. Add the cooked chicken pieces to the pipian sauce. Continue cooking for another 20 minutes on med/low heat. Serve with your favorite rice and warm tortillas!