2 red onions, sliced as thinly as possible
juice of 2 lemons and 2 limes
2 t kosher salt
1 1/2 T white sugar
white wine vinegar
Slice the onions as thinly as you can possibly manage and place into a large, non-reactive bowl. Add salt and sugar. Juice citrus over the onions and then add just enough white wine vinegar so that the onions are just barely covered with liquid. Stir well, cover, and set out at room temperature for 3-4 hours. Drain and serve, reserving the lovely pink liquid to pickle/dress more onions or any number of vegetables: jicama, carrots, cucumbers, kohlrabi, etc.
Serve as the topping to any sandwich, on tacos, alongside or in coleslaw, on fish, in a green salad, and so on. This is also great with any citrus: orange, tangerine, kumquat, even grapefruit.