In a medium saucepan over medium heat, bring the water, vinegar, sugar and salt to a low simmer. Stir for about 1 minute, or until the sugar and salt completely dissolve in the liquid. Remove from heat and let cool for about 5 minutes.
Pour the liquid over the onions. Let stand at room temperature for about 1 hour, then cover and refrigerate for up to 2 weeks.
Notes
This is a basic recipe, which is delicious as written, but can also be customized with a variety of flavors and seasonings.
Herbs: add a sprig or two of rosemary, oregano, thyme or dill.
Seasonings: such as bay leaves, a clove or two of garlic, a few peppercorns, ground allspice, dried chili, and ginger.
Vinegars: mix up the vinegar for slightly different flavor. Red wine vinegar, white wine vinegar, apple cider vinegar and rice vinegar all work well.
Sweeteners: granulated sugar is a classic ingredient, but you can play around with other sweeteners such as honey, agave, or maple syrup.
Mexican Pickled Red Onions: add sliced habañero peppers, jalapeño peppers, or even just a pinch of crushed red pepper flakes to make the onions spicy. Use a pinch of ground cumin, oregano, allspice, or bay leaf for traditional Mexican flavor.
Lemon or Lime: replace some of the vinegar with a different acid, like freshly-squeezed lemon juice or lime juice.