Remove the stems and seeds from the peppers, then cut into one-inch pieces. Put them in a large, heatproof bowl. Separate the slices of red onion and add them to the bowl. Then, add the pickling spice and celery seed. You can either mix them in directly with the peppers and onions, or tie them up in a piece of cheesecloth to remove them later.
Pour the apple cider vinegar, water, sugar and pickling salt into a medium saucepan. Stir everything together and bring the mixture to a boil over medium-high heat. Let the mixture boil for one minute.
Pour the hot brine over the peppers, onions and spices, and give everything a quick stir. Let the peppers sit until they cool to room temperature, then cover the bowl and refrigerate it for 24 hours. Remove the spice bag, if you used one, and enjoy the pickled sweet peppers. Kept in the refrigerator, they'll last about one month.
