Wash and trim the zucchini, then slice them one-sixteenth-inch thick; a mandoline works best. Slice the onion very thin as well.
Combine the zucchini and onions in a large but shallow nonreactive bowl, add the salt and toss to distribute. Add a few ice cubes and cold water to cover, then stir to dissolve the salt.
After about 1 hour, taste and feel a piece of zucchini - it should be slightly softened. Drain and pat dry.
Combine the vinegar, sugar, dry mustard, mustard seeds and turmeric in a small saucepan and simmer for 3 minutes. Set aside until just warm to the touch. (If the brine is too hot, it will cook the vegetables and make the pickles soft instead of crisp.).
Return the zucchini to a dry bowl and pour over the cooled brine. Stir to distribute the spices. Transfer the pickle to jars, preferably ones that have shoulders to hold the zucchini and onions beneath the surface of the brine. Seal tightly and refrigerate for at least a day before serving to allow the flavors to mellow and permeate the zucchini, turning them a brilliant chartreuse color.