Many food scientists and writers have realized for quite a while that the first element of that belief is incorrect. The fact is that, like most vegetables, mushrooms have a high water content to begin with. Soaking them in water may add a tiny amount of water to their weight, but compared with their beginning water content, any added amount is inconsequential.The second belief has been more tenacious. Even though the resident geniuses at New York's International Culinary Center and the blog Cooking Issues debunked the myth a couple of years ago, you still find well-respected (but mistaken) authors and chefs perpetuating the mistake.
The Truth about Mushrooms
As ICC's Dave Arnold and Nils Noren explain, a bunch of wet, crowded mushrooms in a pan will start out as (in their words) "a soupy mess." But if you cook them long enough, the water evaporates, at which point they'll brown beautifully without absorbing the oil you're using to brown them. Dry mushrooms, on the other hand, are very porous. When you heat up a pan with oil or butter and add dry mushrooms, they soak up the fat and never let go of it. You end up with browned but greasy mushrooms.
As an added bonus, the "wet, crowded" method also turns out mushrooms with a much more concentrated flavor, especially noticeable with common cultivated mushrooms. (While I wouldn't hesitate to try this method with wild varieties, I have no first-hand experience doing so.) When we first learned about this method, my teaching partner and I did a side-by-side comparison for a cooking class, cooking half a pound of mushrooms this way and half a pound the traditional way. In a blind taste test, all the students and our assistants preferred the mushrooms cooked the wet, crowded way.
Here's the method (for close-up images, click on the photos):
1. Begin by cleaning the mushrooms.
When they're clean, trim and cut the mushrooms. I almost always quarter them rather than slice, but that's just personal preference. Keep in mind that with this cooking method, the mushrooms will lose volume and end up quite a bit smaller than they start out (check out photo 4 if you don't believe me).
2. Add water, fat and salt.
Choose a saute pan that will fit all the mushrooms two or three deep. Remember, you want them crowded for this method. Add enough water to barely cover the mushrooms. For eight ounces of mushrooms, add about a tablespoon of butter or oil and a teaspoon of kosher salt. Put the pan on a burner turned to high and bring the water to a boil.
3. Keep the mushrooms at a boil.
At this point, you can ignore the pan and go on with dinner prep; you're just waiting for all the liquid to evaporate. As long as you're in the kitchen, your ears will tell you when you need to pay attention to them again.
4. Listen for the sizzle.
This is the point where you can season the mushrooms if you wish. I often deglaze the pan with a little wine or dry sherry, and depending on how I'm using the mushrooms, I might add minced garlic, or a pinch of dried or fresh thyme. If I'm dividing the mushrooms for use in two dishes, I remove the half I'm saving for later and then season only what I'm using immediately.