What does “crowding mushrooms” mean

There's a great experiment on Cooking Issues that deals with this very problem. The general advice is to not crowd the pan because of the concomitant release of water; however, the guys found that doing this is actually beneficial because, although a lot of water is initially released, by the time the liquid has eventually evaporated the mushrooms have collapsed and cook far better in the oil that's left. Here's the full explanation:

To make the test really severe, I decided to cook the soaked mushrooms in one batch in an extremely crowded pan, and the dry mushrooms in 3 batches with plenty of room. We weighed out identical amounts of salt and oil (this is the crucial part) and began cooking.

As we expected, the soaked crowded mushrooms formed a soupy mess in the pan. The dry mushrooms didn’t stew and cooked quickly. Here is where it got weird. The dry mushrooms ended up absorbing all the oil. In fact, I had underestimated the amount of oil they needed. They wanted more. I couldn’t add any more oil, however, because it would have ruined the experiment. When the soaked and crowded mushrooms had finally evaporated all their extra water and stated to sauté, they didn’t absorb all the oil. When they were finished, a significant amount of oil was left in the pan. They looked as good and tasted better and less oily than their dry cousins –by a lot.

Our explanation: While the mushrooms are boiling off their water, they aren’t absorbing oil. By the time the boiling stops they have already collapsed, so they aren’t as porous as a raw mushroom and don’t want to absorb oil. The dry mushrooms start absorbing oil from the get-go.

What I do now is this:

• Add the all the chopped mushrooms to a pan over a medium heat with some salt and water and cover.

• The mushrooms steam and release and ton of their own water: allow to cook for a good 5-10 minutes.

• Drain the mushrooms - reserving the released mushroom water - and dry the pan before returning it to a high heat.

• When the pan is very hot, add a few tablespoons of oil (depending on the volume of cooked mushrooms) and throw in the mushrooms.

• When they are nicely browned, deglaze the pan with the mushroom water

The reason for the two stage process is that I found if you wait for the water to evaporate naturally the residue from the dissolved solids in the water can stick to the pan and burn while the mushrooms are frying. By covering the pan initially, the water doesn't evaporate and can be siphoned off with any dissolved solids (that now can't get stuck to the bottom of the pan) and then reintroduced right at the end. As the pan is so hot it only takes a minute or two for all the water to evaporate and any new fond created by actually sauteeing the mushrooms gets used as well.

Dehydrated mushrooms usually have a really intense flavor, and are a bit tougher than fresh mushrooms. Reconstituted they can be use in risotto, filled pasta, duxelles for an enhanced mushroom flavor.