Sweet and Spicy Canned Onion Marmalade

Prepare waterbath canner, 6½-pint jars & lids, keeping warm until needed.Cut ends off onions and peel; slice in half and cut each in half again, lengthwise (or in thirds if onions are big). Turn halves and cut into ¼" slices widthwise (as pictured above). Place slices in an 8-qt. measuring cup - you should have 6 cups of onion slices.Add prepared onions, apple juice, vinegar, garlic, salt, pepper, mustard and red pepper flakes to an 8-qt. stockpot. Gradually stir in pectin and then add butter if using. Bring to a boil that can't be stirred down over high heat, stirring constantly.Add honey and sugar; return to a full boil and let boil for 1 minute, stirring constantly. Remove from heat (skim foam if needed).Ladle hot marmalade into prepared ½-pint jars one at a time, leaving ¼-inch headspace. Wipe rims, attach lids, and place in canner.Place lid on canner and bring to a gentle, steady boil - process for 10 minutes, maintaining steady boil the entire time. Turn off heat, and remove jars to a towel-lined surface to cool for 24 hours without disturbing. Check lids for seal before labeling and storing in a cool, dark place (with rings removed).