In a small saucepan, combine the mirin and sake. Bring to a boil over high heat and boil briefly, 15-20 seconds. Reduce heat to low and whisk in the miso, stirring constantly until the paste is incorporated into the liquid.
Add 1 tablespoon of the Asian Tea Rub and brown sugar, continuing to stir until the sugar is dissolved. Remove the pan from heat and add the sesame oil. Allow the marinade to cool completely.
Rinse the halibut fillets and pat dry. Transfer the fish to a 1 gallon zip-lock bag and pour in the marinade. Seal the bag and lay it flat in a baking dish or on a tray. Refrigerate for 1-2 hours or overnight.
When you are ready to cook the fish, preheat the broiler and a lightly oiled cast iron pan. Transfer the fish to the hot pan, and sprinkle the remaining Asian Tea Rub on top. Broil 1-3 minutes until the surface of the fish begins to brown and blister.
Turn off the broiler and finish the fish in a 400 degree oven until it flakes easily and is cooked through. It will take about 10 minutes total cooking time, including the broiling phase.
Serve with rice and vegetables for a sumptuous meal.