Wash cucumbers and zucchini and cut off the ends. Slice crosswise into 1/8-inch slices. Toss in a large bowl with the salt and onion slices; cover with about 4 to 6 cups of ice cubes. Cover and let stand for 4 hours or refrigerate overnight.
Prepare the boiling water bath. Add water to a large canner with rack and heat to about 180°. The water should be high enough to be at least 1 inch above the filled jars.
Wash jars thoroughly and heat water in a small saucepan; put the lids in the saucepan and bring almost to the boil; lower heat to very low to keep the lids hot.
Drain the cuke/zuke mixture in a large colander and rinse with cold water.
Mix nigella seeds, mustard seeds,, dill seeds and distribute among the jars.
Place garlic, one leaf per jar if using (for crispness).
In a large pot (nonreactive) over medium heat, combine the remaining ingredients and bring to a boil. Add the drained cucumber mixture and bring to a boil. With a slotted spoon, loosely pack the vegetables in prepared jars. Ladle the hot liquid into jars.
With a clean damp cloth wipe the rims of the jars then cover with 2-piece jar lids. Adjust the screw on rings firmly but do not over-tighten.
Place filled in the prepared boiling water bath, adding more hot water as needed to bring the water up to about 1 inch above the jars. Bring the water to a boil. Cover and continue boiling for 10 minutes.
Lift the jars out of the water and place on a rack to cool for 12-24 hours. Remove rings and label. Give it a week for flavors to meld.