Wash and trim off the ends of the zucchini and peel the onions. Thinly slice the zucchini and the onion. A mandoline works great for this.
Place the zucchini and the onion in a shallow bowl and sprinkle the salt over them and toss to coat. Add a few ice cubes and some cold water, stir to dissolve the salt. Let it sit for one hour.
While the zucchini and onions are resting, make the brine by combining all the brine ingredients in a small saucepan. Bring to a simmer and let simmer for 3 minutes. Remove from the heat and let it come to room temperature.
After an hour, check the zucchini. It should be slightly salty and softened. Drain adn dry with paper towels or in a salad spinner.
Place the zucchini and onions in a jar, one with shoulders will help keep the zucchini submerged. Pour over the brine. Place a lid on the jar and store in the fridge.
Let them chill for at least one day.
Notes
Note - the sodium content is higher than it probably is since the salty brine is drained off. The recipe says these keep almost indefinitely. I try to use mine within 6 months.