A red onion is a is a cultivar1 of the onion. It differs from the more common yellow onion in the following ways:
  • The skin and internal layers of the onion are a purplish red.
  • The skin is looser and thinner.
  • The taste is different, somewhat sweeter, mainly.
  • It spoils faster than a yellow onion. Stored in a dry, cool place, it might last a few weeks2. Mine last on average 2 weeks.
  • It is normally a bit more expensive than a regular onion.

Red onions can be used as a substitute for yellow onions. This is especially nice in say, a salad, as the bright purple makes the dish look better. When sautéd, the difference isn't as visible. The slightly sweeter, more mellow taste is especially good in applications where the onion is eaten raw. It is noted that the specific variety of onion you are eating matters as well: onions are normally grown locally, and the red onion you get in Berlin might be from a different cultivar than the one you get in New York, and as such taste differently.

While there are differences in taste and color, a red and a yellow onion are very similar and you can probably get away with substituting one for the other if you have to. The difference is much smaller than for instance the difference between a yellow onion and a shallot.

Because talking about food can make one hungry, I'll add a simple red onion recipe. I haven't tried it (got it from3), so I can't comment on the taste.

Ingredients:

  • 12 medium-sized red onions
  • 6 garlic cloves.
  • Some fresh thyme
  • 180 g (a little less than 1 cup) of butter
This should serve 6.

Preheat oven to 180 degrees C (350 F). Remove the loose skin from the onion. Remove the bottom of the onion so it can stand. Make 2 deep, crossing cuts in the top of the onion, so it consists of four pieces that are still joined at the bottom. Put them in an baking dish. Put some garlic and thyme in the cuts. Mix butter and the rest of the thyme and the garlic and distribute this mixture over the onions. Push the butter deeply into the onion. Add salt and pepper to taste. Bake for 40 minutes.

In summary, red onions are a cultivar of onions. They are quite similar to yellow onions, and one could usually substitute one for the other, apart from the fact they red onions are bit sweeter and are, obviously, red. The red color is nice in a salad.

Sources:

  1. http://en.wikipedia.org/wiki/Red_onion
  2. http://www.groentenenfruit.nl/veggipedia/Rode%20ui
  3. http://www.iensrecepten.nl/recept/bereidingswijze/oven/gebakken-rode-uien-tijm-knoflook-oven

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