Preheat oven to 425 degrees F (220 degrees C).
Add Mirin (1 cup) and White Miso Paste (1/2 cup) to a medium sauce pan and put on medium-low heat. Stir until smooth and the miso paste has dissolved.
Place each of the Cod Fillet (4) on a lined sheet tray and sprinkle with Salt and Pepper (to taste). Cover each fillet with the miso/mirin mixture evenly. Bake in the oven for 5-10 minutes depending on thickness.
To make the tomato salad, mix the White Miso Paste (2 tablespoon), Butter (2 tablespoon), Honey (1/2 teaspoon), Horseradish Root (1 1/2 teaspoon), Fresh Basil (2 tablespoon), Fresh Chives (2 tablespoon), salt and pepper (to taste) in a bowl. Toss with the Cherry Tomato (3 cup).
Serve the tomato salad on top of the fish.