Preheat oven to 375°.
Cut ribs in serving size portions and arrange in a foil-lined roasting pan; sprinkle with salt and pepper and top with sliced onions. Roast for 1 hour.
Meanwhile, combine the remaining ingredients in a blender; process until the mixture is smooth.
Put the ribs and onions in the slow cooker; top with the sauce.
Cover and cook on LOW for 7 to 8 hours, until very tender. Serves 4.