Makes 1 quart; recipe can be halved or doubled.
2 lbs small zucchini
1 small yellow or white onion
2 cups cider vinegar
1/4 cup honey
1 tsp celery seeds
1 tsp turmeric
1 tsp dry mustard
1/2 tsp kosher salt
Cut unpeeled zucchini into thin slices, like cucumbers. (If you have a mandoline with an adjustment dial, set it to 1/4-inch.) Peel onion and slice thin. (Also on the mandoline, if you're using one.) Place vegetables in a large stainless steel or glass bowl.
In a stainless steel saucepan, combine remaining ingredients. Bring to a boil and pour over vegetables. Let stand 1 hour at room temperature.
Pour vegetables and liquid back into a stainless steel saucepan. Return to heat, bring to a boil, and cook 3 minutes. Pour into hot, scalded jars. Cover tightly and refrigerate.