Cut the roast in half and place it in a large ziptop bag. In a small bowl, combine the water, soy sauce, sesame seeds, molasses, vinegar, green onions and garlic powder. Pour half of the marinade into the bag with the roast. Seal the bag and make sure the whole roast gets coated with the marinade. Refrigerate overnight. Also refrigerate the reserved marinade.

Place the roast in a 5 quart slow cooker and discard the marinade. Add the reserved marinade to the slow cooker. Place the cover on the slow cooker and cook on high for 1 hour. Reduce the heat to low and cook an additional 6-8 hours or until the meat is tender.

Remove the roast from the slow cooker; cover to keep warm. Skim the fat off of the remaining cooking juices. Transfer the liquid to a saucepan and bring to a boil. In a small bowl, whisk together the cornstarch and cold water. Gradually stir the cornstarch mixture into the saucepan. Cook and stir for 2 minutes, or until thickened. Serve over the pork with additional sliced green onions.