These are always on the plate next to the grilled hamburger on a focaccia bun at Zuni Café in San Francisco, along with plenty of garlicky aïoli. Truly a match made in taste heaven, and so easy to do at home.
1 pound small to medium zucchini, green or gold
1 small yellow onion
3 Tablespoons kosher salt
2 cups cider vinegar
1 cup sugar
1-1/2 teaspoons dry mustard
1-1/2 teaspoons crushed yellow or brown mustard seed
1 scant teaspoon ground turmeric
Wash and trim the ends from the zucchini. Slice very thin, about 1/16 inch, using a sharp knife or a mandolin, either straight across or on a diagonal. Peel, halve and slice the onion as thin as the squash. Mix the squash and the onion in a large bowl and toss with the salt. Cover with cold water, stir to dissolve the salt, and add a few ice cubes. Let stand for 1 hour. This step is very important for making a crisp pickled squash.
Drain the vegetables and taste a piece of zucchini. If it seems too salty for your taste, rinse the vegetables under running water and drain again. Place the vegetables on clean kitchen towels and pat dry. Return them to the dry bowl.
Mix the vinegar, sugar, dry mustard, mustard seed and turmeric in a non-reactive saucepan and simmer slowly for 3 minutes. Set aside until warm to the touch.
Add the brine mixture to the vegetables and stir with a wooden spoon to mix well. Spoon the vegetables into small pickle jars and top off with the liquid. Seal tightly and refrigerate for at least a day to allow the flavors to mingle and penetrate. The pickles should be a lovely chartreuse color. They will keep for several weeks in the refrigerator, or for longer storage you can process them in a boiling water bath according to the directions in a good canning book.