This soup is based on the classic Thai dish known as tom yum goong; the secret to making a great one is to put all your effort and love into making a great stock. I encourage you to use dried lobster mushrooms here, as their rehydration liquid, along with the toasted shrimp shell stock, makes a fine base for the soup. Extra bonus: The rehydrated lobster mushrooms retain a touch of chewiness that makes for a great textural contrast. — Shroom, by Becky Selengut
Cleaning Lobster Mushrooms
Lobsters can get quite dirty, especially on their concave tops; use the tip of a knife and a brush (the pros use toothbrushes to clean the mushrooms, and you can too). The knife and brush will get the lion’s share of the dirt off, but you might still need a wipe with a damp towel or a quick rinse with the faucet’s spray attachment to get rid of the rest. Trim off any small brown or dark reddish spots.
Rehydrating Mushrooms 101
You might think it would be as simple as dunking in hot water, but there is a technique for rehydrating mushrooms, especially those that tend to be dirty or gritty. Refer to the video on rehydrating mushrooms at ShroomTheCookbook.com.
For Dried Wild Mushrooms: These can be very gritty, sandy, or dirty, especially morels, porcini, and black trumpet mushrooms. I recommend a two-step rehydration method.
Step 1: Put the dried mushrooms in a small bowl, pour a generous amount of warm tap water over the top, wait 5 minutes, and then aggressively agitate them in this water. (This important step ensures that any grit hiding in or on the mushrooms comes out.) Carefully lift them out, discarding the water left behind (unless you are working with porcini, then see Notes).
Step 2: Now place the mushrooms in a new container, such as a quart-size heatproof pitcher or glass measuring cup, a quart-size canning jar, or a French coffee press. (A tip I learned from Connie
Green’s fabulous book The Wild Table is to rehydrate mushrooms in a tall container, as it allows plenty of room for the hot clean water to circulate around the mushrooms, while the dirt, sand, and grit can settle to the bottom, well below where the mushrooms are floating. I find that a French coffee press is perfect. Use the plunger to submerge the mushrooms just under the boiling water.) I find that you get the most consistent results when you boil the water first before pouring it over the mushrooms. Using hot tap water for this longer step may result in mushrooms that can be anywhere from soft to still brittle. Use at least 2 cups boiling water per ½ ounce of dried mushrooms. If you don’t have a French press and the mushrooms are floating above the surface of the water, place a small cup or plate on them to keep them submerged. Different types of mushrooms take varying amounts of time to rehydrate, typically anywhere from 10 to 30 minutes, or in the case of chanterelles, seemingly never (which is why I don’t recommend using or buying dehydrated chanterelles). When the mushrooms are soft, make sure to lift them up and out of the soaking liquid, squeezing any liquid out of the mushrooms back into the soaking vessel. Avoid getting any sediment back on the mushrooms.
Notes: The thriftiest of us save and use the soaking liquid from all mushrooms, but it’s the wild morels, porcini, and black trumpet mushrooms that kick off the most flavorful liquid, in my opinion. For porcini mushrooms (where the rehydration liquid is arguably more flavorful than the pieces of mushrooms themselves), you’ll want to save each round of the soaking/cleaning water, and carefully pour it through a coffee filter or fine-mesh strainer set over a bowl. Use all of that flavorful liquid in stock or sauce making. For morels and black trumpets (that you didn’t clean and dry yourself), I advise getting rid of the first round of soaking liquid, as it can be terribly gritty.
FOR DRIED CULTIVATED MUSHROOMS: Dried cultivated mushrooms, such as shiitakes or maitakes, rarely have much dirt or grit to speak of, so I recommend just one soak (follow Step 2). Nonetheless, it’s always a good idea to strain the rehydration liquid before using, even with cultivated mushrooms.
Recipes and instructions from Shroom: Mind-bendingly Good Recipes for Cultivated and Wild Mushrooms by Becky Selengut/Andrews McMeel Publishing, LLC.
If using dried lobster mushrooms, rehydrate as directed below. Save the rehydration liquid and chop the rehydrated mushrooms into medium dice. Clean fresh lobster mushrooms as directed below and cut into medium dice.
Place 2 tablespoons of the coconut oil in a soup pot over high heat. Add the shrimp shells and toast until lightly browned and nutty smelling, about 5 minutes. Add the carrot, celery, and onion and sauté until lightly browned, 8 to 10 minutes more. Deglaze the pot with the white wine, making sure to scrape up any bits stuck to the pot. Prep your lemongrass. If the stalks are whole, chop off the top two-thirds and compost them (they’re tough and don’t have the intense flavor of the bottom third). Smash the bottom third with the side of a knife to help release the essential oils. To the stock, add the smashed lemongrass, cilantro stems, kaffir lime leaves, 2 split serrano chiles, and ginger. Add the lobster mushroom rehydration liquid, if using dried mushrooms, and enough of the water to equal 6 cups of liquid total. Bring the stock to a boil over high heat, then lower to a simmer and cook for 30 minutes.
Strain the stock through a fine-mesh sieve into a container, pressing on the ingredients. Clean the soup pot and return to the stove over medium-high heat. Add the remaining 1 tablespoon coconut oil to the pot along with the lobster mushrooms. Add the pinch of salt and sauté until brown and caramelized, 8 to 10 minutes. It’s OK to get a little bit of char on the mushrooms; the flavor adds a lovely smokiness to the broth. Add the diced tomato and stock. Bring back to a boil over medium-high heat. Simmer for 10 minutes to infuse the flavor of the mushrooms into the soup. Add the raw shrimp and immediately turn off the heat and cover (the shrimp will poach using this technique and won’t overcook). After 2 minutes, lift the lid and add the lime juice, fish sauce, remaining 1 teaspoon salt, and sugar. Adjust the amounts of these final seasonings to your taste. Garnish each bowl with sliced serrano chiles, basil, and cilantro leaves.