On paper towels, pat tofu dry on all sides. Cut tofu crosswise into 1/2-inch thick slices. On small plate, place cornstarch.

Heat nonstick wok or large skillet over medium heat until hot. Add 2 tablespoons oil and swirl to coat pan. One at a time, dip tofu slices into cornstarch to coat cut surfaces and place in wok. Fry until well browned on one side-about 5 minutes . Turn tofu slices and fry other sides until well borwned. Transfer tofu to center of large serving platter; cover with foil and keep warm.

Add broth and gingerroot to wok and heat to boiling. Add bok choy; cover and cook until tender and most of broth evaporates-about 5 minutes.

Meanwhile, prepare Sauce. In small bowl, combine vegetable broth, oyster-flavored sauce, soy sauce, cornstarch, sugar and sesame oil.

With slotted spoon or pancake turner, remove bok choy and place, cut sides down, around rim of platter with tofu; cover and keep warm. Discard ginger slices and any remaining broth.

Reheat wok over high heat until hot. Add remaining 1 tablespoon oil and the mushrooms. Stir-fry mushrooms until softened-about 3 minutes. Restir Sauce and pour over mushrooms; cook, stirring constantly, until thickened and bubbly. Transfer mushrooms from Sauce and place over tofu. Pour Sauce over all. Garnish with green onions, if desired.