1. Place first 8 ingredients in a large heavy-duty zip-top plastic bag; seal bag. Turn bag to blend marinade. Place roast and onion in bag; seal bag, turning to coat. Marinate in refrigerator 24 hours, turning bag occasionally.
2. Remove roast from marinade, reserving marinade. Place roast in a 5-quart electric slow cooker. Strain reserved marinade through a sieve into a bowl, reserving 1 1/2 cups; discard remaining liquid and solids. Pour 1 1/2 cups strained marinade over roast. Cover and cook on LOW for 5 hours or until roast is tender.
3. Remove roast from slow cooker; cover and keep warm. Add gingersnap crumbs to liquid in slow cooker. Cover and cook on LOW for 8 minutes or until sauce thickens; stir with a whisk until smooth. Serve sauce with roast. Garnish with parsley, if desired.