Sprinkle kosher salt and freshly ground black pepper on both sides of the salmon fillet.
Thinly slice the onion and cut the carrot into julienne strips.
Cut off the bottom of the shimeji mushrooms and shiitake mushrooms. Break up shimeji mushrooms with your hands and thinly slice shiitake mushrooms.
Cut the chives into small pieces.
Prepare two 12” x 12” (30 x 30 cm) sheets of aluminum foil. Thinly spread the butter in the center on both sheets.
Place half of the onion slices and put the salmon on top, skin side down.
Put two kinds of mushrooms and then carrot julienne strips on top of the salmon.
Add ½ Tbsp sake and ½ Tbsp butter on top.
Bring the top and bottom aluminum foil over the salmon and fold a few times. Then bring the sides together and fold a few times to make sure the foil is tightly closed and no opening. Repeat the same process for the second salmon.
To Cook on the Stovetop
Place the two sets of salmon in foil on a frying pan and cover with a lid (no need to add water). Cook the salmon over medium heat for 2 minutes, then medium-low heat for 10 minutes. If you have 1 to 1 ½ inch-thick salmon, you will need to cook for 15-18 minutes (Open the foil to check the doneness).
To Bake in the Oven
Alternatively, you can bake at a preheated 425ºF (218ºC) oven for 15 minutes. For a convection oven, reduce cooking temperature by 25ºF (15ºC). If you have 1 to 1 ½ inch-thick salmon, you will need to cook for 18-20 minutes. Carefully open the foil to check the doneness, if you're not sure.
Carefully open the aluminum foil to enjoy the salmon, sprinkle chives and drizzle ponzu or soy sauce on top. Enjoy!