1. Combine sugar, soy sauce and pepper flakes in a small bowl, cover and set aside.
2. Coat a 5-quart slow cooker with nonstick cooking spray, combine potatoes and onion in slow cooker. Pour pineapple with its juice over top. Place pork loin filet on top and sprinkle with black pepper. Cover and cook on high for 4 hours (low: 8 hours).
3. Place pork on a cutting board and let stand 10 minutes.
4. Add butter to potato mixture in the slow cooker. Using a potato masher or fork, mash until well blended.
5. Slice or coarsely shred pork. Spoon the potatoes on a serving platter, top with half of the pork. Stir the soy sauce mixture and spoon over pork and potatoes. Reserve the remaining pork for the Asian Pork and Peanut Salad recipe.