Step 1

Liquid: Heat water in either a kettle, the microwave, or on the stove until steaming hot. Stir in sugar and salt until dissolved. Stir in vinegar.

Step 2

Onion: Thinly slice the red onion, either with a mandoline slicer (for ultra-thin slices) or by hand (for slightly thicker, crunchier pickled onions).

Step 3

Combine: Add onions to a lidded non-reactive container (like a glass jar or ceramic vessel - I used a 16-oz mason jar). Pour liquid over onions so that they are covered (you may not need all of the liquid). Let cool to room temperature, then seal shut and transfer to the refrigerator. (Let pickle for at least 30 minutes before digging in.)