Prepare the teriyaki sauce by whisking together soy sauce, mirin, sake, sugar, minced garlic, and grated ginger in a bowl.

Heat sesame oil in a large skillet or wok over medium-high heat. Add the thinly sliced beef and cook until browned.

Pour the prepared teriyaki sauce over the beef. In a small bowl, dissolve cornstarch in water and add it to the skillet to thicken the sauce. Cook until the sauce coats the beef.

Serve the teriyaki beef over cooked rice. Garnish with chopped green onions and sesame seeds before serving.