Boil the Base: In a small saucepan over high heat, combine the rice wine and 1/4 cup mirin. Bring to a rolling boil for 1 to 2 minutes to boil off the alcohol.
Mix the Marinade: Lower the heat to its minimum setting. Add the white miso paste and sugar. Whisk constantly until the mixture is completely smooth and the sugar has dissolved.
Cool Down: Remove the saucepan from the heat and let the marinade cool to room temperature. Do not add warm marinade to raw fish.
The Long Soak: Place the Mahi Mahi filets in a gallon-sized bag. Pour in the cooled marinade, squeeze out the air, and seal. Refrigerate for up to 2 days for maximum flavor penetration.
Prep the Sides: Prepare your white rice. For the Bok Choy, sauté the chopped stems in a pan with oil, ginger, and garlic. Add the leafy greens and soy sauce in the final 2 minutes. Finish with a splash of mirin for sweetness.
Sauté the Fish: Remove the Mahi Mahi from the bag and gently pat dry with a paper towel. Heat a pan over medium heat with a little oil.
The Sear: Place the fish in the pan. Cook for 3 to 4 minutes per side. Watch closely; the sugars in the miso will caramelize quickly.
The Oven Finish (Optional): If the filets are thick and not yet opaque in the center, transfer them to a 400°F oven for 5 minutes to finish cooking without burning the glaze.
Serve: Plate the fish over a bed of white rice with a side of the sautéed Bok Choy.