1-1/2 pounds red onions, including papery skins
6 pounds underripe or tart apples
3 1-inch cinnamon sticks
4-1/2 cups water
3-3/4 cups apple cider vinegar (5% acidity)
4-1/2 cups granulated sugar
Pull the papery skins from the onions, setting them aside; remove and compost the root ends. Using a mandoline or sharp knife, thinly slice the onions, setting aside enough of the thinnest to equal 1 cup. Zest the lemon, adding the zest to the pile of sliced onions. Chop the remaining onions, onion peels, lemon, and apples, retaining the apple peels, cores, and seeds, and add them into a wide, 6-quart or larger pot. Drop in the cinnamon sticks, pour in the water and vinegar, and bring the mixture to a boil over high heat. Lower the heat to medium high, and cover loosely. Cook for about 30 minutes, until the apples break down and their peels separate from the pulp. Stir as needed to prevent burning.
Strain the juice as you would for Sweet Pepper Jelly: Set a fine-mesh or cheesecloth-lined colander over a large bowl, pour in the hot mixture, and let the juice drain for at least 30 minutes; stir occasionally, but don’t press down on the pulp. Set the colander on a plate and measure out the juice; you should have about 6 cups. If you come up short, return the colander to the bowl, stir in some hot water, and let the solids drain until you have the desired amount of liquid.
Clean the pot and add the juice and sugar; stir in the onion slices and lemon zest. Bring to a boil over high heat and cook, stirring often, for about 15 minutes, or until the marmalade sets when you test it.
Ladle into hot half-pint jars, leaving 1/4-inch headspace. Process in a boiling water bath for 5 minutes, plus your altitude adjustment. Makes about 7 half-pint jars.
Tips & Tricks
- Although you could use other onions, red ones produce the most colorful marmalade. The onion skins give even more color, so use as many as you can—including those from red onions you might be peeling for another recipe (see below).
- A zester that cuts citrus peel into long, thin strips matches the shape of thinly sliced onions. You can use a grater-style zester instead, but expect to see and taste flecks of peel.
- If you’re growing apples, pick the ones you need for this and other jamming recipes, like Tomato–Apple–Basil Jam, before the first frost sweetens them; they’ll release more pectin into the mix. If you’re buying apples, choose tart ones, like Granny Smiths.
- Unlike dense marmalades such as Fall Marmalade and Rhubarb–Orange–Ginger Marmalade, this recipe produces a jelly-clear marmalade with suspended solids. To ensure it sets, bring the mixture to 8°F above the boiling point of water at your altitude.