How to Make It
Preheat the oven to 425° or prepare your grill or campfire for cooking over moderate heat. In a small saucepan, whisk the soy sauce with 1/3 cup of the mirin and the sugar and bring to a boil over high heat. Simmer over moderate heat for about 5 minutes, stirring occasionally, until the sauce is reduced by about half and lightly coats the back of a spoon.
Meanwhile, line a baking sheet with foil and set the unagi on it. Sprinkle with the remaining 1 tablespoon of mirin. If using a grill or campfire, sprinkle the unagi with mirin before wrapping loosely in foil. Cook for about 6 to 8 minutes, until heated through.
Divide the steamed rice among 4 bowls. Cut each unagi fillet in half and divide among the bowls, skin side down. Drizzle each bowl with some of the warm sauce and sprinkle with some sansho pepper or Sichuan peppercorns, if desired. Serve.
Unagi can be found in Asian grocery stores. Smoked trout can be substituted.