To begin, combine the diced onions and butter in a heavy Dutch oven.
Cook over low heat for 1-3 hours, low and slow, until the onions are very soft and golden. Stir frequently to prevent scorching. The onions will reduce in volume considerably.
Once the onions are ready, add the remaining ingredients.
Increase the heat to a very gentle simmer and cook until it has reduced to a nice, thick jam consistency.
Canning the Maple Onion Jam
Ladle the hot jam into hot 4 ounce canning jars. Allow for 1/4 inch of head space.
Remove air pockets with a bubble wand and wipe the rims of the jar with a damp cloth that is dipped in white vinegar.
Place a lid and ring on the jar and place it in the canner. Repeat with all the jam.
Process the jars for 10 minutes in a hot water bath or steam canner. Adjusting for elevation if needed. (See note in the article above.)
When the jars have processed, transfer them to a lined surface in a draft free location. Cool for 12-24 hours.
Remove the rings, test the seals, and label the jars. Store in your pantry for up to a year. If any jars didn't seal, put them in the fridge to use right away.