In a saucepan, bring the water, red wine vinegar, sugar, red pepper flakes, onions, and garlic to a low boil. Stir and cook for 7-10 minutes or until the sauce reduces by about half, thickens, and becomes sticky. When finished set the sauce aside.
In a large pan, fry the zucchini in a few tablespoons of olive oil (coat the pan) until well browned on both sides (about 2-3 minutes per side). Spread the zucchini out to drain on paper towels, and sprinkle with kosher salt as they come out of the frying pan. Work in batches, using more olive oil as required until all the zucchini discs are fried.
Layer the zucchini in a flat dish and pour the vinegar sauce on top. If the sauce doesn't touch all of the zucchini, flip the top layer after 15 minutes or so. Let the zucchini sit for about 30 minutes to absorb the sauce, then refrigerate for at least 1 hour to chill.
The zucchini will taste much better in a couple of hours after they absorb the flavor of the sauce. Serve the zucchini with a sprinkle of fresh parsley. Enjoy!