Add the sake to a small saucepot and bring to a boil over high heat. Reduce the heat to medium so that the sake remains at a heavy simmer and cook for 8 minutes to concentrate the flavor.

Add the kasu, mirin, soy, sugar, and miso to the sake and whisk to incorporate completely. Return to a simmer and cook until the kasu is completely tender and the marinade has reduced, about 10 minutes. 

Transfer to a blender and process until perfectly smooth. Refrigerate until completely chilled. 

Brush the halibut filets, with a thick layer of the marinade on all sides and wrap with plastic before marinating for 8-12 hours. 

Wipe the excess marinade off the fish and season lightly with salt. 

Sear in a pan over medium heat with just enough neutral oil to coat the bottom of the pan.

Flip the fish and finish in the oven at 350°F for 6 minutes, or just until the fish flakes easily. Serve over a bed of Sake Risotto.