Heat the vinegar just to boiling, or in the microwave for 1 min.
Add salt and sugar, stir to dissolve.
Place red onions and other spices in the jar.
Add hot mixture to the jar, cover and leave to cool to room temperature.
Once cool, cover tightly and refrigerate for up to 2 weeks.
Video
Notes
The key to perfect pickled red onions is to slice the raw red onions thin. Unless you have excellent knife sills, I would suggest a Mandolin slicer or using the slicer blade on your food processor. This also ensures that the onions are all sliced to the same thickness for even pickling.
For a different flavor profile, feel free to use rice vinegar instead of white vinegar.
You can also use pure maple syrup instead of white sugar if you prefer.
Throw in some mustard seeds or a bay leaf if you prefer.
For some heat, toss in some red pepper flakes.
This is a technique for quick pickles. Making them this way means they are not shelf stable. These must remain in the refrigerator while being stored.
Great for all types of onions they are a great way to use up onions before they go bad.