In a large bowl (at least 6 qt.), combine vegetables, onions, salt, and 2 1/2 quarts water. Cover and chill at least 12 hours or up to 24 hours. Drain, rinse well, and drain again.
Follow steps 1 through 4 of Canning Instructions, using six pint-size jars (seven if using all cucumbers).
In an 8- to 10-quart pan over high heat, bring vinegar, sugar, mustard seeds, celery seeds, and turmeric to a boil, stirring often. Add vegetable mixture, cover, and return to a boil; reduce heat and simmer, stirring occasionally, until onions are limp, about 2 minutes.
Follow steps 5 through 11 of Canning Instructions, leaving 1/2 inch headspace in each jar; be sure liquid covers vegetables (discard any leftover liquid). Bring water in canner to 180°-185° and process for 30 minutes.
Note: Nutritional analysis is per 1/4 cup drained.