Bok Choy with Bean Curd

1 pound bean curd (tofu)
4 medium stalks bok choy
4 scallions (with tops)
1/4 cup vegetable oil
1 (4 ounce) can button mushrooms, drained (reserve liquid)
3 tablespoons oyster sauce
1/2 teaspoon instant chicken bouillon
1 tablespoon cold water
1 tablespoon cornstarch

Cut bean curd into 3/4-inch cubes; drain thoroughly. Cut bok choy (with leaves) into 1/4-inch slices. Cut scallions into 2-inch pieces. Heat 2 tablespoons of the oil in wok or 10-inch skillet over medium-high heat until hot. Cook bean curd, stirring carefully, 3 to 5 minutes. Remove bean curd. Add remaining oil and the bok choy to wok; stir-fry over medium-high heat 2 minutes. Add mushrooms. Add enough water to reserved liquid to measure 1/2 cup. Stir liquid, the oyster sauce and bouillon into vegetables; heat to boiling.

Mix water and cornstarch; stir into vegetable mixture. Cook and stir until thickened, about 1 minute. Stir in bean curd and scallions. Heat until hot.