To Make the Marinade: combine all ingredients in a small saucepan, bring to a boil, and then lower heat and simmer for 5 minutes. Strain to remove solids and refrigerate until ready to use (you will be using 2 tablespoon of the marinade in the dressing).
To Make the Dressing: Whisk together the dressing ingredients with two tablesploons of the marinade and set aside.
At least two hours before serving: place the fish in a baking dish and pour on 1/3 cup marinade.
To Finish the Dish: Preheat broiler on high.
Start boiling water for the soba.
Line four plates with spinach.
When the broiler is ready, put in the fish (using an oven-proof pan). Turn after 4 minutes, adding more marinade. Cook for another 4 minutes or until the fish flakes easily and turns opaque.
When the water boils, add the soba and carrots. Be sure to only cook for the time noted in the package direction — do not overcook.
When ready, drain the soba and carrots and rinse with cold water, drain again and set aside in a mixing bowl. Toss with the dressing.
To serve, mound the soba and carrots on the spinach and place the fish in the center.