Remove bones from the mackerel fillets and score the skin in a cross pattern. Sprinkle with salt and let sit for 5 minutes, then pat dry.

Cut the green onion in half, remove the core, and slice thinly. Soak the slices in water for 5 minutes and drain. Mince the core.

Place the mackerel on a microwave-safe plate, sprinkle with sake, and top with minced green onion. Cover loosely with plastic wrap and microwave at 600W for 5 minutes. Transfer to a serving dish and garnish with sliced green onion.