Step 1Place water and salt in a saucepan and warm over moderate heat. When the first bubbles begin to appear, remove from heat. Cover the pot and steep for 30 minutes (or up to overnight). Strain through a fine-mesh strainer. This makes slightly more stock than is required for the remainder of the recipe; refrigerate the stock in an airtight container up to three days.
Step 1Bring a large pot of salted water to a boil. Blanch the tomatillos until soft but still whole, about 5 minutes. Remove from heat and drain; place the tomatillos in a blender with 2 cups cilantro leaves and 1 cup parsley leaves, the green onions, serrano chiles, kombu stock, fish sauce, olive oil and salt. Puree until smooth. Transfer the sauce to a medium saucepan and bring to a boil over high heat. Reduce the heat and simmer until slightly reduced, 10 to 15 minutes.
Step 2Meanwhile, pat the fish dry with paper towels and cut into 3- to 4-inch pieces, seasoning both sides of each with sea salt. Place a large saute pan over moderately-high heat and add enough oil to coat the bottom of the pan. Add the fish in one layer (this may need to be done in batches) and cook until each piece moves easily with a slight prod, 3 to 4 minutes. (If you fuss over the fish too much it will fall apart — resist the urge to move, peek or flip more than once). Using a fish spatula, flip the fish and continue to cook until the fish is opaque throughout, about 2 minutes more.
Step 3Taste the reduced sauce, adjusting seasoning as desired: lemon will add brightness and salt will add interest. If you’d like more heat, slice another chile and serve it on the table.
Step 4Divide the fish among the serving bowls, and ladle the sauce on top. Garnish each with a spoonful of crème fraiche and a scattering of herbs and sliced onion.