Combine the ingredients for the seasonings (1 Tbsp miso, 1 Tbsp sugar, 2 Tbsp sake, 1 Tbsp mirin, 1 ½ Tbsp soy sauce).
Finely chop chives and cut off the stem of shimeji mushrooms. Separate shimeji into smaller pieces with hands.
Cut off the stem of shiitake mushrooms and thinly slice them.
Check to see if your salmon has any bone, and remove if there is any (it’s good to have a pair of fish bone tweezers). Pat dry the salmon with paper towel and season the salmon with kosher salt and freshly ground black pepper.
Sprinkle flour on the salmon and thinly coat it all over the salmon. The flour will prevent the delicious juice leaking from the salmon and it also acts as a thickening agent for the sauce.
Heat the olive oil in a frying pan over medium heat. Place the salmon from flesh side. You can see how far you’ve cooked from the side of the fillet. Don’t burn your salmon using high heat. Cook for 5 minutes.
Flip so the skin side is on the bottom and add the shimeji and shiitake mushrooms. Cook the skin side for 5 minutes.
Then cook the sides of the salmon for 1 minute each. It is completely okay when there is a bit of translucent pink in the middle. The remaining heat will continue to cook the rest of the fish.
Add the butter and saute the mushrooms. Put the mushrooms on the salmon so salmon gets some of the delicious sauce.
Add the seasonings and coat the salmon using a spoon.