1Remove leaves from bok choy stems. Cut leaves into 2-inch pieces; cut stems into 1/4-inch diagonal slices (do not combine leaves and stems). Cut tofu into 1/4-inch slices. Coat tofu with cornstarch. Cut shallots into thin slices.
2Heat wok or 12-inch skillet over high heat. Add 2 tablespoons vegetable oil; rotate wok to coat side. Add 2 pieces tofu; cook 1 minute, turning once. Repeat with remaining tofu, removing tofu as necessary. Return tofu to wok. Add oyster sauce; toss until tofu is evenly coated. Remove tofu from wok.
3Heat wok over high heat. Add 2 tablespoons vegetable oil; rotate wok to coat side. Add shallots; stir-fry 30 seconds. Add bok choy stems and salt; stir-fry 1 minute. Add tofu and bok choy leaves; cover and cook 1 minute.