Cut the tofu in half horizontally, wrap it in paper towels, and microwave at 600W for 2 minutes to remove excess moisture. Coat lightly with cornstarch. Slice the green onion diagonally into 7mm pieces.

Heat salad oil in a frying pan over medium heat. Add the tofu and cook until both sides are golden brown, flipping occasionally. Remove from the pan.

In the same pan, add the seasoning ingredients (soy sauce, mirin, sugar, sake, and water) and bring to a boil. Add the green onion, cover, and simmer for 3 minutes. Uncover, add the beef, and cook until the beef changes color. Push the beef to the side and return the tofu to the pan, cooking until the sauce thickens and coats the tofu.