Pat the pork dry with a paper towel. Combine 1 tablespoon of oil with all the rub ingredients. Season the pork by rubbing the mixture into the meat.
Heat remaining oil in a large pan over medium heat. Sear the pork on all sides until golden browned. Rotate the pork to avoid burning.
Melt butter in the same pan the pork was cooked in while scraping up any bits stuck to the pan. Add garlic and shallot and sauté for 1 minute until fragrant. Stir in the remaining sauce ingredients and bring to a simmer for about 1 minute.
Place the seared pork in a 6-qt slow cooker. Pour the sauce over the pork, add the raisins, cover and cook on LOW heat setting for 2 1/2-3 hours.
Transfer the pork onto serving dish and tent loosely with foil. Let rest for 10-15 minutes.
While the pork is resting, pour the juices from the slow cooker bowl into a medium pot. Bring to a simmer over medium-high heat.
Mix 2 tablespoons of the juices with the cornstarch using a fork to get rid of any lumps. Whisk cornstarch slurry into sauce and let simmer for a good 2-3minutes, or until thickened.
Slice and drizzle with the port raisin sauce.
