2 red onions, cut into 1-inch wedges, root ends left intact
4 clove garlic, smashed
4 lb bone-in beef short ribs (about 6), cut in 3 1/2-inch pieces
1 1/2 cup dark-brown sugar
1 cup low-sodium soy sauce
6 tablespoon rice vinegar
1 tablespoon Sriracha, or other chili sauce
3 cup pineapple, cut into 1-inch cubes (1 pound)
Cooked white rice, for serving
Green onions, chopped, for serving
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.
To store, refrigerate short ribs and cooking liquid in an airtight container for up to 3 days.