In a large pot, add water and kombu. Bring to a boil and remove from heat. Discard the kombu and add the bonito flakes to the hot water. Let flakes steep for 5 to 8 minutes. Strain to remove solids.
Add maitakes to dashi, and check for seasoning. On a small plate, season the fillets, skin side only, with fleur de sel and pepper. Heat a skillet to very high and crisp up the skin side only. The top of the salmon should still be raw.