Use a mandolin to thinly slice the zucchini and onions. In a large mixing bowl, combine the zucchini, onions and 2 tablespoons salt. Gently toss; let stand for two hours. Rinse well and drain.
Whisk together vinegar, sugar, mustard seed, celery seed, turmeric and 1 teaspoon salt in a large skillet or pot with a lid; bring to a boil and simmer for 5 minutes. Add the zucchini and onions, remove from heat, cover, and let stand for two more hours, gently turning the zucchini over to marinate in the pickling solution.
Reheat and boil pickles for just 5 minutes. Cool until warm, and place in hot sterilized jars. Seal tightly and turn upside down for 15 minutes. Turn right side up and you should have a seal. Store in the refrigerator for up to 2 weeks (if they last that long). Makes 3 pints