Season the pork with salt and pepper and add to a dutch oven. Add the white onion, garlic cloves, bay leaf, and enough stock so the pork is submerged. Bring up to a simmer, top with a lid and simmer for 1.5-2 hours, until the pork is tender.
Make the sauce while the meat is cooking. Start by toasting the pumpkin seeds. In a pan over medium heat, add the pumpkin seeds and toast. Stir frequently to prevent burning. When the seeds start to pop and toast, turn off the heat. Allow them to cool and then grind in a spice grinder until they are smooth. Set aside.
In a large pot over medium heat, add a few tablespoons of avocado oil or pork fat. Add the tomatillos, serrano peppers, poblano pepper and garlic cloves and toast. Turn occasionally so they are golden brown on all sides. Season with salt as you go.
Once the items in the pot are golden brown, add the ground pepitas, cilantro, and enough broth from the cooking pork to submerge the ingredients. Season with salt and simmer for 20-30 minutes to develop flavor and thicken.
Once slightly reduced, blend the sauce with a blender or immersion blender. Taste and adjust seasonings accordingly. Once the pork is done cooking, remove the pork from its pot, and add to the pot with the pipian sauce. Stir to coat, then serve and enjoy!
