Combine the miso paste, brown sugar, white rice vinegar, and soy sauce in a bowl. If it’s too thick, gradually stir in 1 to 2 tablespoons of water until it’s the right consistency.
Set out a sheet of plastic wrap or aluminum foil for each salmon filet. Set one filet on each sheet.
Brush the marinade on all sides of the filet until they are thoroughly covered in marinade.
Wrap the filets in the sheets and marinate in the refrigerator for about 30 minutes.
Preheat the oven to 355 degrees F (180 C).
Boil 2-3 cups of salted water in a pot on medium-high heat.
Blanche the snow peas for 8-10 minutes. Remove from heat and cool down in an ice bath.
Line a baking sheet with aluminum foil. Optionally, you can coat the aluminum foil with olive oil to prevent the marinade from sticking to it.
Place salmon on the foil. Cook in the oven for 12-15 minutes until it is flaky but still pink on the inside.
While the salmon is cooking, melt butter in a saucepan over medium heat. Saute shallot for 7 minutes. Add sake and stir well. Optionally, add salt and pepper to taste.
Arrange salmon and snow peas on a plate; top everything with the sake butter. Serve with your favorite rice dish.