Rinse the mushrooms in cold water and drain. Remove and discard the stems. Cut the caps into 1-inch strips.
Rinse the green beans in cold water and drain. Snap off the ends.
In a small bowl, mix the cornstarch and 2 teaspoons cold water.
Heat a wok over high heat. Add the vegetable oil and rotate the wok to coat the sides. Add the garlic, green beans, and salt and stir-fry for 1 minute. Add the mushrooms and continue to stir-fry until mixed. Stir in the oyster sauce. Stir in the chicken broth and heat to boiling. Stir in the cornstarch mixture; cook and stir until thickened, about 30 seconds.
Turn off the heat and remove the green beans with tongs. Arrange the green beans across a platter and top with the mushrooms.