How to Make It
Stir together first 7 ingredients in a 6-quart slow cooker. Cover and cook on HIGH until onions are deeply softened and mixture is slightly soupy, about 7 hours. Remove lid, and continue to cook until most of the standing liquid has evaporated, about 1 more hour. Turn off slow cooker, and stir in syrup and vinegar. Cool mixture 30 minutes. (If mixture still has standing liquid after 1 hour of uncovered cooking, strain into a medium saucepan, reserving onions, and add syrup to liquid in saucepan. Cook over high until reduced to about 1/4 cup, 12 to 15 minutes. Stir vinegar and reserved onions into reduced syrup mixture. Cool 30 minutes.)
Transfer mixture to a food processor. Pulse until chunky but spreadable, about 10 times. Divide evenly among 4 (8-ounce) jars. Cover and store in refrigerator up to 2 weeks.