How to Make It
In a small skillet, toast the sesame seeds over moderate heat, stirring a few times until golden, 3 to 4 minutes. In a small bowl, mix the soy sauce with the rice vinegar, mirin, grapeseed and sesame oils, scallions and ginger. In a large, shallow dish, brush the halibut fillets with half of the soy-ginger dressing. Cover and refrigerate for 30 minutes.
Preheat the oven to 450°. Transfer the halibut to a large, rimmed baking sheet and season with pepper. Bake the fish in the upper third of the oven for 10 to 12 minutes, or until glazed and just cooked through.
Spoon the rice into shallow bowls. Set the halibut fillets on top and spoon the spinach alongside. Pour the remaining dressing over the fish, sprinkle with the toasted sesame seeds and serve.