The other week I made a discovery. This wasn't one of those exciting new-species-of-butterfly or great-shoes-right-in-the-back-of-the-shop-on-the-left type discoveries. No, this was one of those discoveries you make when undergoing a monthly refrigerator purge: several pounds of button and cremini mushrooms I had leftover from shooting a video for pressure cooker mushroom risotto, most of them lightly bruised and past their prime.
I'm sure you've had those mushrooms in the past yourself. The bad news is that once mushrooms start showing discoloration or soft spots like that, they're not really any good for roasting or salads or even pizzas, where those flaws and soft spots make them kinda unappetizing. The good news is that they're still perfectly fine for creamy mushroom soup, where those flaws get blended away.
The soup I made with them is based on my creamy chanterelle soup. Barring a few minor differences, the process is almost identical, the only real difference ends up being the flavor and the cost (button or cremini mushrooms are, oh, 10 times cheaper than chanterelles). This recipe will work with pretty much any variety of mushroom I can think of, so go with whatever you like best.
Flaws are fine, but dirt is not, so the first step is washing the mushrooms. When I was first learning to cook, I was taught that washing mushrooms was a Very Bad Thing. They'll absorb water like a sponge!, they said. They'll never brown right!, they said. Just do what I say!, they said. I was taught to painstakingly brush dirt and debris off of them using a dampened paper towel or a mushroom brush.
But by this late stage in the game, we all know that none of that is really true, right? Everyone from Alton Brown to Cook's Illustrated has performed tests on this and found that while it's true that mushrooms absorb a little bit of water when you wash 'em, that small amount of extra water only takes a moment to evaporate off when you're browning them in a pot—overall you save a ton of time. The best way to do it is in a salad spinner with plenty of water that you drain off with a thorough spin. The only thing you want to make sure of is to wash the mushrooms just before you're going to use them. Washing then storing in the fridge can lead to faster decomposition.
After washing and slicing, I browned the mushrooms in butter. With that chanterelle soup, I sauté the mushrooms with garlic and shallots without browning them to preserve that sweet chanterelle flavor. With buttons or cremini, I like to brown them to deepen their muted flavor. This takes a bit of time. At first, the mushrooms sweat out a ton of liquid and it looks like they're never going to brown. But as that liquid evaporates, you'll start to hear the sound coming out of the pot gradually shift from a murmur-like simmer to the sharper sizzle of a sauté. I tried browning mushrooms to different degrees and found that I liked them best when just lightly browned but still plenty moist and tender, which gave the soup a creamier texture while still producing plenty of flavor.
Once the mushrooms were browned properly, I added some onion and garlic and cooked them down until softened.
As with the chanterelle soup, I added a couple tablespoons of flour to help thicken up the base of the soup, then deglazed the pan with some dry sherry, using a wooden scraper spoon to help scrape up flavorful browned bits from the bottom of the pot. For my liquid, I used a combination of milk and chicken broth, though water or homemade vegetable stock would work fine if you want to keep the dish vegetarian. Just avoid store-bought vegetable stock, which is almost universally bad.
A couple sprigs of thyme and a couple bay leaves also helped boost flavor.
After a short simmer, I fished out the thyme and bay leaves, then puréed the whole thing with an immersion blender before serving it. Given that the whole thing started with past-their-prime mushrooms, it may seem like overkill to gussy them up with minced fresh herbs (classic fines herbes—parsley, chervil, tarragon and chives—are a good way to go if you have them on hand) and a drizzle of extra-virgin olive oil, but I'm the kind of guy who likes to wear fancy hats to that pool party.