Combine the mirin, soy sauce, sugar, ginger, garlic, and cayenne in a small saucepan and bring to a boil. Lower heat and simmer until reduced a thick glaze, 5 to 6 minutes. Remove from the heat.
Set the halibut fillets on a baking dish and sprinkle with cayenne. Paint on both sides with the glaze. Let marinate for 15 minutes.
Bring the glaze back to a boil and boil for 5 minutes. Remove from heat and keep warm.
Place the sesame seeds in a dish and dip the fillets onto the seeds, pressing lightly to coat each side with about 1 teaspoon of seeds.
Heat the oil in a large skillet over medium-high heat. Add the fillets and cook until browned and just done, 3 to 4 minutes per side. Remove from the pan to 4 serving plates. Garnish with the green onions and brush with any remaining glaze. Serve with the Cucumber Noodles.
Yield: 4 servings