Ladle the hot jam into the hot jars, leaving 1/4-inch headspace. Place the lids and bands on top, screwing on the bands just until fingertip-tight. Place the full jars back into the boiling water and process 15 minutes. Allow the jars to sit in the canner 5 minutes with the heat and lid off. Remove from the water and place the jars on a towel.
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Stir onion, salt, and black pepper into sugar mixture. Onions will immediately start to caramelize. Stir frequently over medium heat until sugar has dissolved and onions are browned, about 15 minutes. Step 4. Pour red wine vinegar into onion mixture and simmer until mixture has a jam consistency, about 30 minutes.