Halve the tofu horizontally. Remove excess moisture by pressing it between sheets of paper towels.
Cut the mushrooms into halves and slice thin.
Combine the mushrooms, carrot, potato, bell pepper, leek, egg white, soy sauce, salt and saké in a bowl. Sift 2 tablespoons of the cornstarch over the mixture and stir to mix well. Meanwhile, pre-heat the bro