Season the pork with salt and pepper. In a large, heavy skillet, heat the olive oil over medium heat. Add the pork roast and cook, turning, until browned, about 15 minutes.
Grease the inside of a large slow cooker. Add the onion and celery. Arrange the pork on top and pour in the cider. Scatter the apples and raisins over the pork and sprinkle the sage on top. Cover and cook on low heat until tender, about 5 hours.
Transfer the pork to a cutting board and thickly slice. Skim the fat from the gravy. Serve the pork with the gravy, fruit and vegetables.