I always keep a giant jar of these in my fridge. They are a great topping for almost any dish (egg whites, chicken, sandwiches, salads, etc) and they only take 10 min to make.
1 large red onion
1 cup apple cider vinegar
1/2 cup red wine vinegar
1/4 cup white wine vinegar
1 tsp salt
Fill a large pot with water and boil
While you are waiting for the water to boil, slice the onion into thin strips, then place in a colander in the sink
When water is boiling, slowly pour over the sliced onion. Let the onion drain, then toss with the salt.
Put the onion in a large glass jar. Add the vinegar and refrigerate for at least 2 hours before using.
Feel free to play around with the vinegar ratios. Most recipes call for more red wine vinegar, less apple cider vinegar, and some sugar. I have found that with the ratios I use I don't need to add sugar and I prefer the taste.