Combine first 6 ingredients in a small bowl, and set cornstarch mixture aside.
Heat 2 teaspoons vegetable oil in a stir-fry pan or wok over medium heat. Add ginger and garlic; stir-fry 30 seconds. Add mushrooms, asparagus, and leek; stir-fry 3 minutes. Add cubed tofu; stir-fry 4 minutes. Stir in cornstarch mixture. Bring to a boil; cook 1 minute. Serve over rice.