Searing and browning the pork in the slow cooker guarantees even more flavor. Slideshow: More Crock Pot and Slow Cooker Recipes
Score the skin of the pork with a sharp knife. Make 1-inch deep incisions with a pairing knife all over the pork and insert 1 slice of garlic in each incision. Season the pork with 1 1/2 teaspoons salt and 3/4 teaspoon pepper. In a 5-to 6-quart crock pot, heat the oil over high heat until hot. Sear the pork butt, turning once, until browned, 6 to 8 minutes total. Add the stock and cover the crock pot.
Simmer the pork butt on low, covered, until the meat is very tender, 7 to 8 hours. Shred the pork, discarding the bone, then strain the cooking liquid into a fat separator and pour the fat back into the crock pot. Heat the fat over high heat until hot, then cook the shredded pork in the fat, stirring frequently, until browned, 10 to 12 minutes. Season with salt and pepper to taste and serve with tortillas.