Lightly sprinkle salt and pepper over pork roast. Set aside.
In a large pan, heat 2 tablespoons vegetable oil. Fry onions for about 2 minutes. Add dried rosemary and fry for another minute. Transfer to slow cooker insert.
Add the remaining tablespoon of vegetable oil to the pan. Brown pork roast on all sides. Transfer to slow cooker insert.
Pour water into pan. Add dried cranberries, cinnamon stick, dark soy sauce, sugar, and salt. Bring it to a boil and allow sugar to dissolve. Turn off heat and pour sauce over pork roast in the slow cooker.
Cover and cook on high for 6 hours. After 4 hours, turn the pork roast. Pour cornstarch mixture into the slow cooker and continue to let it cook for another 2 hours.