Originally from Lianna Krissoff in her book Canning For a New Generation
Makes approximately 6-7 pint jars
4 pounds yellow summer squash (another source suggested zucchini, which I may also try)
8 pounces sweet onion (about 1 medium)
¼ cup plus 1 tablespoon pure kosher salt
2 tablespoons cumin seeds
1 tablespoon yellow mustard seeds
1 tablespoon hot red pepper flakes
1 teaspoon ground cumin
6 cups cider vinegar (5% acidity)
2 tablespoons mild honey
7 cloves garlic
7 small fresh serrano chiles
Scrub the squash and cut it into ¼ inch rounds. Cut the onion in half lengthwise and thinly slice it into half-circles. Put the squash and onion in a large bowl and sprinkle with the ¼ cup salt, tossing to combine. Cover with a layer of ice cubes and refrigerate for 8 hours or overnight. Pick out any remaining ice, and rinse under cold water. Drain well. (The original recipe says at this point to toss the squash with the cumin seeds, mustard seeds, red pepper flakes, and ground cumin and set aside. I’ve found I lose some spice in the bowl this way because even when I drain it well it’s still wet. I’ve taken to mixing the spices in a separate bowl and dividing them among the sterilized jars before adding the squash and onions.)
Prepare water bath for canning. Ladle boiling water from the canning pot into the bowl with the lids. (Here again, I’ve since begun keeping the lids at a simmer on another eye in a little saucepan, a la Marissa McClellan from Food In Jars.) Using a jar lifter, remove the hot jars from the canning pot, carefully pouring the water from each one back into the pot, and place them upright on a folded towel. Drain the water off the jar lids.
Working quickly, pack the squash, onion, garlic, and chiles into the jars (not too tightly). Ladle the hot vinegar mixture into the jars, leaving ½ inch head space at the top. Use a chopstick to remove the air bubbles around the inside of each jar. Use a damp paper towel to wipe the rims of the jars, then put a lid and ring on each jar, adjusting the ring so that it’s just finger-tight. Return the jars to the water in the canning pot, making sure the water covers the jars by at least 1 inch. Bring to a boil, and boil for 15 minutes to process. Remove the jars to a folded towel and do not disturb for 12 hours. After 1 hour, check that the lids have sealed by pressing down on the center of each; if it can be pushed down, it hasn’t sealed, and the jar should be refrigerated immediately. Label the sealed jars and store.