Easy Bok Choy Stir Fry

1 1/2 tablespoons soy sauce
1 tablespoon dry Sherry
1 teaspoon sesame oil
1 teaspoon cornstarch
2 teaspoons vegetable oil
3 large garlic cloves, minced
1 tablespoon minced peeled fresh ginger
1/8 teaspoon dried crushed red pepper
3 1/2 cups thinly sliced trimmed bok choy
1 5-ounce can sliced water chestnuts, drained
4 green onions, cut into 1-inch pieces
10 ounces extra-firm tofu, drained, cut into 3/4-inch pieces (optional)

Combine first 4 ingredients in small bowl; mix well. Heat vegetable oil until very hot in heavy large wok or skillet over medium high heat. Add garlic, ginger and crushed red pepper. Stir-fry until aromatic, about 30 seconds. Add bok choy and stir-fry until just wilted, about 2 minutes. Mix in water chestnuts and green onions and stir-fry until onions are tender, about 1 minute. Add tofu and lightly stir-fry until tofu is just heated through, about 2 minutes. Pour over soy mixture. Stir-fry until liquid boils and thickens, about 1 minute. Serve.