How to Make It
In a small saucepan, combine the soy sauce, sake, mirin, sugar and lemon zest and bring to a boil. Simmer over low heat until fragrant, about 5 minutes.
In a small bowl, dissolve the cornstarch in the lemon juice. Whisk into the saucepan and bring to a boil, then cook over low heat for 1 minute, stirring. Let the teriyaki sauce cool.
Light a grill. Lightly oil the grate. Pat the tofu dry with paper towels. Place in a shallow baking dish and brush with the sesame oil. Grill the tofu over a medium-hot fire, basting with the teriyaki sauce, until heated through and browned, about 6 minutes. Transfer to a platter. Drizzle with the remaining sauce, sprinkle with the scallions and sesame seeds and serve.