pickled onion: red onion, cider vinegar, red wine vinegar, sugar, salt, pepper, water.
Halve and peel a red onion, and slice each half as thinly as possible, parallel to the root end.
Use the back of a wooden spoon (or similar implement) to gently pack sliced onion into a jar. For best results, choose a jar you can fill completely with onion.
Mix red wine and cider vinegars in equal proportion so that you have more than enough liquid to fill the onion jar halfway. Whisk in sugar to taste until dissolved. (I like about 2 tablespoons for a medium onion.)
Fill onion jar a little more than halfway with vinegar mixture. Top off with water until onions are fully submerged. Let sit at room temperature for at least one, and preferably four hours or more before use. Refrigerate leftovers for a week or more.
(Taste the onions halfway through the pickle to see if you like the level of acidity. If not, pour out some liquid and replace either with water or with vinegar.)