2 pounds firm fresh zucchini, cut into 1/4-inch slices
2 small onions, sliced
1/4 cup canning salt
3 cups white vinegar
2 cups sugar
2 teaspoons mustard seed
1 teaspoon celery seed
1 teaspoon turmeric
Combine zucchini and onions in a large bowl; sprinkle with canning salt and cover with cold water. Let stand 2 hours; rinse and drain.
In a large saucepan, bring remaining ingredients to a boil. Pour over zucchini and onions; cover and let stand 2 hours.
Transfer to a large saucepan. Bring to a boil; reduce heat and simmer 5 minutes. Carefully ladle hot mixture into four hot half-pint jars, leaving 1/2-in. headspace. Remove air bubbles and adjust headspace, if necessary, by adding hot mixture. Wipe rims. Center lids on jars; screw on bands until fingertip tight.
Place jars into canner with simmering water, ensuring that they are completely covered with water. Bring to a boil; process for 15 minutes. Remove jars and cool.