Layer zucchini, onion, and salt in a large glass container; add water to cover. Let stand 2 hours. Rinse and drain zucchini and onion 3 times.
Combine mustard seeds and celery seeds; tie in a cheesecloth bag. Combine spice bag, vinegar, sugar, and turmeric in a large stainless steel saucepan; bring to a boil. Add zucchini mixture. Remove from heat; let stand 2 hours. Bring mixture to a boil. Reduce heat; simmer 5 minutes, stirring gently. Discard spice bag.
Pack vegetables into hot sterilized jars; cover with hot syrup, leaving 1/3-inch headspace. Remove air bubbles. Cover at once with metal lids, and screw bands tight. Process zucchini in boiling-water bath 15 minutes.