Season the goose thighs generously with salt and pepper and rub well into the meat. Heat a skillet or pot over medium-high heat and sear the thighs well on both sides until deeply browned. Transfer the seared goose thighs to the slow cooker.

In the same skillet, add the quartered onion, dried red chiles (seeds removed unless you want extra heat), garlic, cumin seed, coriander seed, peppercorns, cloves, and oregano. Sauté until fragrant and lightly toasted. Add a small splash of stock (about one cup) and simmer for 3–5 minutes to allow the chiles to rehydrate and the onion to soften.

Transfer the mixture to a blender and blend until smooth. Pour the blended mixture into your pot or a bowl and stir in the remaining stock, crushed tomatoes, and apple cider vinegar until well combined.

Pour the sauce over the goose thighs in the slow cooker. Add the cinnamon stick and bay leaves, cover, and cook on low for 8-10 hours, until the meat is fall-off-the-bone tender.

Remove the goose thighs from the sauce, discard the bones or keep for stock for another day, and shred the meat. Toss the shredded meat in some of the sauce on a separate plate if desired, and remove the bay leaves and cinnamon stick from your slow cooker.

Heat a skillet over medium heat. Dip a corn tortilla into the birria sauce, coating both sides lightly, and place it in the hot skillet. Top with shredded cheese, shredded goose, diced onion, cilantro if using, and a drizzle of sauce. Fold the tortilla closed and cook until crispy and golden on both sides.

Serve immediately with extra birria sauce on the side for dipping, or Americanize it with sour cream and salsa.