Prep your jars by preparing a water bath canner and bringing the water to a simmer with six half-pint jars warm.

Peel and slice the onions into 1/4-inch pieces to yield roughly 6 cups of onion slices.

Combine onions, apple juice or cider, cider vinegar, minced garlic, salt, black pepper, dry mustard, and optional red pepper flakes in an 8-quart stockpot.

Heat over high heat, stirring until it reaches a full boil with thick bubbles forming.

Add honey and brown sugar, stir to combine, and return to a boil for 1 minute.

Carefully fill each warm jar leaving a 1/4-inch headspace and wipe the rims clean.

Process the jars in simmering water for 10 minutes, then cool for 5 minutes before removing.

Allow the jam-marmalade to cool completely at room temperature for 24 hours before checking seals and storing.