Nutrition per serving may change if servings are adjusted.
1 pound zucchini, (2 medium), cut into 1/4-inch slices
1 pound yellow squash, (2 medium), cut into 1/4-inch slices
3 medium onions, sliced
2 tablespoons plus 3/4 teaspoon pickling salt, divided (see Note)
2 cups crushed ice
1 cup plus 2 tablespoons cider vinegar
1/2 cup packed light brown sugar
1/2 cup water
1 1/2 teaspoons mustard seeds
1/2 teaspoon whole cloves
1/2 teaspoon turmeric
1/2 cinnamon stick
1/4 teaspoon celery seeds
1/8 teaspoon crushed red pepper
Combine zucchini, squash, onions, 2 tablespoons salt and ice in a large bowl. Place a plate on the vegetables and weight it with a 5-pound object, such as a bag of sugar. Let stand at room temperature for 3 to 4 hours.
Drain vegetables in a colander. Do not rinse.
Combine vinegar, brown sugar, water, mustard seeds, cloves, turmeric, cinnamon stick, celery seeds, crushed red pepper, drained vegetables and remaining 3/4 teaspoon salt in a large nonreactive saucepan. Bring to a boil over high heat. Remove from heat.
Transfer vegetables and liquid to a 2-quart glass or ceramic bowl or jar. Cool to room temperature; cover and refrigerate for at least 4 days to allow flavors to develop.
Make Ahead Tip: Cover and refrigerate for up to 1 month.
Note: Pickling salt, also called canning salt, produces a clear brine. Do not substitute any other salt.