Prepare a boiling water bath and 5 regular-mouth 1-pint/500 ml jars that have been sterilizedas well as the lids and screw rings. Only dip the lids in hot water for 30 seconds, do not boil them.
Combine the chopped bell pepper, zucchini, and onion in a large, nonreactive pot. Stir in 2 cups of the apple cider vinegar and bring to a simmer over medium heat. Cook until the vegetables have cooked down, about 30 minutes.Stir occasionally.
Drain the vegetables and return to the pot this is done so that your pH is acidic enough to process in a water bath. Add the remaining apple cider vinegar, sugar, salt, mustard seed, celery seed, and red pepper flakes. Bring to a simmer and cook for 5 minutes.
Ladle the relish into the prepared jars, leaving 1/2 inch/12 mm of headspace. Gently tap the jars on a towel-lined countertop to help loosen any bubbles before using a wooden chopstick to dislodge any remaining bubbles. Wipe the rims, apply the lids and rings, and process in a boiling water bath for 10 minutes. Add any additional time if you live in a high altitude.
When the processing time is up, remove the canning pot from the heat and remove the lid of the canning pot. Let the jars sit in the pot for an additional 5-minutes. This helps to prevent the relish from reacting to the rapid temperature change and bubbling out of the jars.
Remember to leave your jars on the counter for 12-24 hours undisturbed. I like to place my jars on a clean towel on the counter so they don’t get any shock. The vaccum seal is formed during the cooling process. When they have cooled label and date your jars.Check your jars and seals before storing. Check them again before eating. It there is mold or the seal is popped up Don’t even Taste the contents but throw the jar away.
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