Thinly slice the onions (white and/or red). If you have a mandolin, this is an excellent time to use it, but it is not essential.
Rub the onion slices with ½ tablespoon of salt and rinse in cold water.
Juice the limes.
Thinly slice the radishes ,cucumber ,and zucchini.
Place the cucumbers, radishes, and zucchini, half of the chopped cilantro leaves, and hot peppers (optional) in a glass bowl and pour the lime juice over the ingredients. Sprinkle with a little bit of salt. To minimize the acidity of the limes, you can add a couple of ice cubes.
Cover and refrigerate for about 1-2 hours.
If garnishing with sweet potato, cut it in half put them to boil, or steam.
Taste any veggies and add additional salt if needed.
Top with the remaining freshly chopped cilantro.
Serve immediately with your choice of sides and garnishes.