Slice one red onion into very thin strips using a sharp knife or mandolin. Break apart the slices with clean hands. Add to a clean quart sized jar.
Make the brine in a medium liquid measuring cup. Whisk together the water, vinegars, salt and sugar until the salt and sugar are fully dissolved. This should take about 30 seconds to a minute of vigourous whisking.
Pour the brine over the onions. Add an extra splash of water or vinegar as needed to have one inch of liquid above the onions.
Refrigerate for at least 2 hours. The liquid will turn bright pink and the onions should be more tender. Enjoy on tacos, grain bowls and in sandwiches. For best quality, keep refrigerated for 2 weeks.
Salt: I use table salt but kosher salt and sea salt both work.
Can I pickle these? No! This recipe does not work for pickling/long-term preserving, just quick pickling.
Spices and seasonings: Add a pinch of ground cloves and orange zest for warming flavors. Other ideas: 3-4 crushed garlic cloves, fresh dill, 1-2 teaspoons crushed red pepper flakes (spicy!), 1-2 teaspoons whole black pepper.
Vinegar: Try white wine vinegar, red wine vinegar, or rice wine vinegar in place of cider vinegar for different flavors.
Sweetness: Adjust sweetness up to ¼ cup sugar or maple syrup. After that, it will start getting too syrupy.