1.Thirty minutes prior to cooking, take tofu out and wrap in paper towel. Place a plate on top of tofu then add a heavy can on top to help drain the moisture. Discard the liquid.
2.In a small bowl mix sake, mirin, and soy sauce. Set aside. Cut tofu into squares 1/2-inch thick. Set aside.
3.Add oil to a nonstick pan on medium-high heat. Meanwhile, coat each tofu piece with potato starch. Once the pan is hot, add the tofu to the pan.
4.Cook until tofu is crispy and golden brown on the bottom. Flip and cook the other side. When both sides are browned pour in the sauce mixture.
5.Flip the tofu and coat with the sauce on all sides. Sprinkle bonito flakes on top and then turn off the heat. Transfer to a plate and sprinkle green onions on top. Serve immediately.