Sear the meat: Heat the oil in a large skillet over medium-high heat. Pat the pork loin dry and sear for 3 minutes per side until deeply browned. Transfer to the slow cooker.
Mix the sauce: In a bowl, combine the undrained pineapple, soy sauce, brown sugar, rice vinegar, garlic, ginger, and red pepper flakes. Pour over the pork.
Slow cook: Cover and cook on Low for 6 to 7 hours or High for 3 to 4 hours, until the internal temperature reaches 145 degrees Fahrenheit.
Rest the meat: Remove the pork to a cutting board, tent loosely with foil, and let it rest for 15 minutes before slicing.
Thicken the glaze: Pour the juices and pineapple from the slow cooker into a saucepan and bring to a boil. Whisk the cornstarch and cold water together, then stir into the boiling liquid. Cook for 3 minutes until thick.
Serve: Slice the rested pork and pour the thickened pineapple sauce generously over the top.
