Place onions in a 4- to 5-quart slow cooker. Place ribs on top of onions. In a blender, combine preserves, chipotle pepper, apple cider, and vinegar. Cover and blend until smooth. Reserve 1-1/4 cups raspberry mixture for sauce; cover and chill until needed. Pour remaining raspberry mixture over ribs in cooker.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours.
For sauce, in a medium saucepan, bring reserved raspberry mixture to boiling; reduce heat. Simmer, uncovered, for 5 minutes. Using tongs, transfer ribs and sliced onions to a serving dish; discard cooking liquid. Spoon some of the sauce over ribs and onions. Pass remaining sauce. Makes 4 to 6 servings.