These are the best refrigerator pickles ever. The key is to use a mandoline and slice very, very thinly. I can’t stand how good these are. I always double this and use the extra brine for cucumber pickles, too. You can re-use the brine for months. I sometimes add dill (fresh or dried) and garlic (fresh or powdered).
1 pound zucchini, washed, trimmed, and very thinly sliced (1/16th of an inch)
1 medium yellow onion, very thinly sliced
2 tablespoons fine sea salt
2 cups apple-cider vinegar
1 cup sugar
1 1/2 teaspoons dry mustard
1 1/2 teaspoons slightly crushed yellow mustard seeds
Scant 1 teaspoon ground turmeric
Combine zucchini and onion in a large shallow bowl. Add salt; toss to combine. Add a few ice cubes and enough cold water to cover, stirring until salt dissolves. Let stand, at room temperature, until zucchini are slightly salty and softened, about 1 hour.
Drain, removing any remaining ice cubes. Dry thoroughly between two towels, or spin, a few handfuls at a time, in a salad spinner (excess water will thin the flavor and spoil the pickle). Rinse and dry the bowl. Return the zucchini and onion to the dry bowl.
In a saucepan, combine vinegar, sugar, dry mustard, mustard seeds, and turmeric, over medium heat; simmer for 3 minutes. Remove from heat, and let stand until just warm to the touch.
Pour the cooled brine over the zucchini-and-onion mixture, stirring to combine. Transfer to sterilized jars. Cover, and refrigerate for 1 to 2 days before serving. Will keep for about 3 months, refrigerated, in an airtight container.