If canning, prepare waterbath canner, 6 half pint jars & lids, keeping warm until needed. If storing in fridge or freezer, prepare jars or other freezer containers.
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 1/4 inch slices widthwise (as pictured in tutorial). Place slices in an 8 quart 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 quart or larger 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).
To Waterbath Can
Ladle hot marmalade into prepared half pint jars one at a time, leaving 1/4-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 let jars cool in canner 5 minutes before removing jars to a towel lined surface to cool for 24 hours. Check lids for seal before labeling and storing in a cool, dark place (with rings removed).
To Store in Fridge or Freezer
Let the marmalade cool for a bit and ladle into freezer safe containers leaving a 1-inch headspace if freezing.
Notes
*You can't substitute maple syrup for the honey in canning (as I previously stated- I recently learned this!). Use cane sugar if you can't use honey.High Altitude Adjustment: If processing at an altitude of higher than 1000 feet above sea level you'll need to adjust the processing time according to this chart.Storage:
Canned: will last 18 months in a cool, dark pantry.
Fridge or Freezer: You don't have to can this - it will keep for 2-3 months in the refrigerator, 6-12 months in the freezer (leave more headspace in the jars for freezer expansion, though).
Serving suggestions:
Use as a glaze for meats like chicken, ham and pork, as well as seafood (my favorite is shrimp).
Top grilled steak, pork, chicken and seafood.
Add on top of Boursin cheese or cream cheese as an appetizer.
Mix into sour cream as a dip for crackers, pretzels, or chips.