Maple-Onion Jam

We recommend pouring jam in spoonfuls of 4 jars (8 ounces each), but you can also choose 8 jars (4 ounces each) to further enhance the flavor.
Combine the first 7 ingredients in a 6-quart slow cooker. Cover and cook on high until the onions are softened and the mixture is slightly soupy, about 7 hours. Remove the lid and continue cooking until most of the remaining liquid has evaporated, about 1 hour more. Turn off the slow cooker and add the syrup and vinegar. Let the mixture cool for 30 minutes. (If there is still liquid left in the mixture after 1 hour of uncovered cooking, strain into a medium-sized saucepan, reserving the onion, and add the syrup to the liquid in the pan. Cook over high heat until you have about 1/4 cup, 12 to 15 minutes. Combine the vinegar and remaining onion with the remaining syrup. Let cool for 30 minutes.)Transfer the mixture to a food processor. Beat until a smooth mass is obtained, capable of spreading, about 10 times. Spread evenly over 4 8-ounce cans. Cover and refrigerate for up to 2 weeks.