Place onions, garlic, and ginger in a 5- to 6-quart slow cooker. Top with short ribs in a tight layer. In a medium bowl, whisk together brown sugar, soy sauce, vinegar, and Sriracha and add to slow cooker. Cover and cook on high until ribs are almost tender, 4 hours.
Add pineapple and cook until pineapple is tender, 1 hour.
With a slotted spoon, transfer ribs, pineapple, onions, and ginger to a platter and tent with foil. With a ladle, skim fat from cooking liquid.
Serve ribs and pineapple mixture with rice; drizzle with some cooking liquid and sprinkle with scallions.