How do you cook onions for 15 minutes without them coloring? Is this a mistake in the recipe in terms of timing or in terms of indicators?

I typically just add a very small amount of kosher salt to the onions. This causes the onions to sweat a bit, and the moisture on the exterior will keep the onions from browning.

In addition to the low heat, do make sure to stir the onions so that the bottom ones don't color. I've made that soup before -- it's great -- and onions that've caramelized will be just fine.

You can definitely saute onions for 15 minutes without coloring them. I took a look at the recipe, and think the purpose is just to soften them and remove some acid, before they go into the soup base.

Just use a non-stick pan and two or so tablespoons olive oil (or canola) in a warmed frying pan. Stir the onions a few times on Low heat (not Medium Low - use Low or somewhere between Simmer and Low). You’re just getting the raw flavor out of the onions, not browning them for presentation or to caramelize them for depth of flavor. It’s important that your onions are very thinly sliced or diced, so that they warm through quickly in the frying pan. Don’t be too concerned – they’re going into a blender or food processor next to get further shredded.

It sometimes helps to soak the onions in cold water for 30 minutes to remove some of bite.

So many words about onions. Basically, the bottom line is that you don’t want dark, caramelized, long-cooked thicker onions. Because that would prevent you from getting the amazing, pale, spring-green color of this soup, in February.

Low and slow! Give it a try, very low, covered, they wont brown in that time period. It's a great way to slowly and fully carmelize your onions, starting low and slow, then increasing the heat after 15-25 minutes.