Place a large colander or sieve over a large bowl. Place the zucchini and onion slices in the colander, sprinkle with salt and toss with your hands. Let sit for 30 minutes to allow the salt to extract excess moisture. Rinse the zucchini and onion under cold running water. Spread out on a clean dishtowel and pat dry.
Prepare your jars for canning: wash jars well in dish soap, rinse and place right side up in a large pot in water to completely cover by 1”–2“. Bring water to a boil. Reduce heat to a low and simmer, covered, at least 10 minutes. Leave the jars in simmering water until you are ready to fill them.
Prepare lids: wash and rinse the flat lids and the round screw-on bands. Set bands aside; place the lids in a small pan and cover with boiling water. Simmer for 5 minutes; let lids rest in the hot water until ready to use.
Place a large, heavy pot on the stove top. Put in the vinegar, sugar, celery seed, mustard seed and turmeric. Turn heat to medium high and stir while bringing to a boil. The sugar will be completely dissolved.
Add the zucchini and onions. Stir gently and cook for 10 minutes. The zucchini and onions will become limp.
Remove the jars from the hot water using tongs. Do not pour out the hot water from the pot; this water will be used for the hot-water bath. Ladle the hot pickles into the hot jars, leaving 1/2” space at the top. Take the clean lids from their sterilizing water and place lids on the jars, rubber side down. Screw the bands on the lids until hand-tight. Using tongs, lower the filled, sealed jars into the hot water. Bring to a boil and process in the boiling water bath for 10 minutes.
Remove the jars and let them cool on the countertop. As they cool you will hear a pop from each jar. When cool the center of the lids will be rigid and not flex when you push down on them.