In a small saucepan, combine the soy sauce, mirin, sake, sugar, garlic, and ginger. Bring to a boil, then reduce the heat to low and simmer for 10 minutes.

In a small bowl, whisk together the cornstarch and water until smooth. Add the mixture to the saucepan and whisk to combine. Simmer for an additional 2-3 minutes, or until the sauce thickens. Remove from heat and set aside.

Preheat a grill or grill pan to medium-high heat. Season the mackerel fillets with salt and pepper.

Grill the mackerel fillets for 5-6 minutes on each side, or until cooked through.

Brush the teriyaki sauce onto the fillets during the last minute of cooking on each side.

Serve the mackerel fillets with steamed rice and vegetables, and garnish with sesame seeds and chopped green onions.