How to Make It
In a large bowl, toss the zucchini and onion with 2 tablespoons of the kosher salt. Cover with ice water and let stand until just softened, about 45 minutes. Drain the zucchini and onion well and pat dry.
Meanwhile, in a medium saucepan, combine the remaining 1 tablespoon of salt with the vinegar, sugar, mustard seeds, mustard powder, turmeric and 1/2 cup of water. Bring to a boil, stirring to dissolve the sugar; let cool completely.
Transfer the zucchini and onion to a 1-quart glass jar and pour in enough brine to cover. Seal with the lid and refrigerate overnight before serving.