Place the potatoes, pineapple, 1/4 cup of the reserved pineapple juice, the brown sugar, cinnamon, nutmeg, butter and molasses into a slow cooker. Toss to mix; ensuring the potatoes are completely coated.
Cover and cook on high for 2 1/2 to 3 hours, stirring gently every 30 minutes. Whisk the cornstarch and the remaining pineapple juice together in a bowl to make a slurry. Add the slurry to the potatoes, mixing gently but thoroughly, to combine. Simmer the potatoes for 3–5 minutes.