Halve block of tofu horizontally, then cut each piece into 12 squares or triangles; set aside. In small bowl, combine soy sauce, sugar, cornflour and 1⁄2 cup water; set soy sauce mixture aside.
In a wok or a large non-stick frying pan, heat 2 teaspoons oil over medium heat. Add spring onions, ginger and garlic and cook for 1 minute or until tender.
Stir in shiitake and button mushrooms. Add 1⁄2 cup water; cover and cook, stirring occasionally for 5 minutes or until mushrooms are tender. Transfer to a bowl.
Add remaining 2 teaspoons oil and bok choy to pan; stirring frequently, cook for 5 minutes or until bok choy is tender.
Return mushroom-spring onion mixture to pan and add tofu. Stir soy sauce mixture to recombine, add to pan. Cook for 2 minutes or until tofu is heated through and vegetables coated with sauce.