Two to three days before serving, combine the sake and mirin in a medium saucepan and bring to a boil over high heat. Let it boil for 20 seconds to allow the alcohol to evaporate, then turn the heat down to low.

Add in the miso paste and whisk together. Once the miso has dissolved, turn the heat up to high again.

Add in the brown sugar while whisking to ensure it doesn't burn. Once the sugar has fully dissolved, remove from heat and allow to cool completely.

Using paper towels, pat the fish fillets dry thoroughly.

Slather the fish with the miso marinade and place in a non-reactive dish or bowl. Cover tightly with plastic wrap.

Place the dish in the refrigerator and allow to marinate for 2-3 days. If pressed for time, you can marinate for 20 minutes at room temperature instead.

Pre-heat the oven to 400°F. Heat an oven-proof skillet over high heat on the stovetop.

Lightly wipe off any excess miso on the fillets, but do not rinse it off.

Grease the skillet with oil and place the fish inside.

Cook until the bottom of the fish browns and blackens in spots, about 3 minutes. Flip and continue cooking until the other side is browned, 2 to 3 minutes.

Transfer the skillet to the oven and bake for 5 to 10 minutes. The fish will be done when it is opaque and flakes easily.

Optional: serve with pickled ginger.