Bring the mirin and sake to a boil in a small non-reactive saucepan over high heat. Reduce heat and add miso, stir to dissolve the miso. Add sugar, increase heat and continue to cook until sugar is completely dissolved. Set aside to cool.
If the plan is to use your oven, you can take the skin off the fish. If grilling leave it on to help keep the fish together.
Once cooled, spread the marinade on the fish evenly. Cover tightly or seal it in ziplock bag. Let it sit for 24-48 hours
Before cooking, wipe excess marinade off of fish
To cook: Preheat oven to 400 degrees or outdoor grill.
Oven method: Roast at 400 degrees for 8-10 minutes. Finish fish by broiling a couple of minutes to hear the edges and brown the outside.
You can also panfry the fish first to brown and create those crispy edges before finishing the fish in the oven for 7-10 minutes or until fish flakes.
Grill method: Place fish skin side up on grill and cook on indirect heat for 8-10 minutes.
Garnish with green onions and serve with pickled ginger if you like.
Notes
You can use any firm fish with this marinade and method, salmon, bass or halibut would also work.