Preheat oven to 425F (218C.) Cut unagi fillet into 4 pieces and place them on the baking sheet lined with aluminum foil. Sprinkle sake on the surface of the unagi and bake in the oven for 6 to 8 minutes until heated.

In a small sauce pan, mix all the ingredients of the sauce and bring it to a boil. Simmer for about 10 minutes until slightly thickened.

Place steamed rice in an individual bowl, sprinkle about a tbsp of sauce over the rice, and top with cooked unagi on top. Pour some sauce on top of the fish. Sprinkle on Sansyo Japanese pepper on top (optional.) Serve immediately.