How to Make It
Preheat the oven to 375°. In a medium saucepan, bring the mirin and sake to a simmer. Add the marudaizu shoyu and brown sugar and simmer over low heat for 10 minutes, stirring occasionally. Add the chiles and let cool to room temperature.
Pick out the chiles and transfer 1 cup of the sauce to an 8-inch square baking dish. Refrigerate the remaining sauce for another use. Add the halibut to the sauce in the baking dish and marinate at room temperature for 20 minutes, turning several times.
Remove the fish from the marinade and blot dry; reserve the marinade. Heat the oil in a large nonstick, ovenproof skillet. Add the halibut and cook over moderately high heat for 2 minutes. Turn the fillets. Transfer the skillet to the oven and cook for about 5 minutes, until the fish flakes with a fork.
Meanwhile, pour the reserved marinade into a small saucepan and boil over moderately high heat until slightly reduced, about 5 minutes. Transfer the halibut to plates and drizzle with the mirin sauce.
Marudaizu shoyu is soy sauce made from whole soy beans, and its deeper flavor is superior to more commercial sauces made with defatted soy meal. It's available at Asian markets.