Step 1

Cut 1½ lb. skinless salmon fillet into 1" pieces. Place in a large bowl and season with 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt.

Step 2

Trim tough stems off 8 oz. mixed mushrooms (such as shiitake, oyster, maitake, and/or nameko). Slice shiitake mushrooms ¼" thick. Tear oyster, maitake, and/or nameko mushrooms into small clusters. Add to bowl with salmon.

Step 3

Trim root end from ½ medium onion; peel and discard skin. Coarsely chop onion and transfer to a medium bowl. Smash, peel, and coarsely chop 3 garlic cloves. Transfer to bowl with onion.

Step 4

Using a spoon or vegetable peeler, remove skin from one 3" piece ginger. (A spoon works well to reach all the nooks and crannies.) Thinly slice into rounds and add to bowl with onion.

Step 5

Cut 1–3 red or green Thai or serrano chiles in half lengthwise (use 1 for mildly spicy, 2 for medium, or 3 if you want sweat beading on your forehead) and place in a small bowl.

Step 6

Remove tough outer layers from 2 lemongrass stalks, revealing the softer, pale yellow stalk. Trim off top two thirds of stalk and discard (or save to use for tea). Slice lemongrass into thin rings, then finely chop; add to bowl with chiles.

Step 7

Heat 1 Tbsp. virgin coconut oil in a large Dutch oven or other heavy pot over medium heat. Add onion, garlic, and ginger and cook, stirring occasionally, until softened and fragrant, about 3 minutes. Add chiles and lemongrass, 8 makrut (Thai) lime leaves or zest of 2 limes, two 13.5-oz. cans unsweetened coconut milk, remaining 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and 3 cups water. Bring to a boil over medium-high heat, then reduce heat until soup is barely bubbling (medium-low heat or lower) and gently simmer, stirring occasionally, until flavors come together, 8–10 minutes.

Step 8

Add salmon and mushrooms to pot, increase heat to medium-high, and return to a boil. Reduce heat to medium-low and simmer, until salmon is just cooked through and starting to flake and mushrooms are softened, 8–10 minutes. Gently stir in 2 Tbsp. fish sauce and 1 tsp. sugar. Slice 1 lime in half and squeeze in juice.

Step 9

Ladle soup into bowls. Cut remaining lime into wedges. Tear or coarsely chop cilantro leaves with tender stems. Serve soup with lime wedges and cilantro on the side.