In a large bowl, blend cornstarch, garlic, 1 teaspoon ginger, and 2 tablespoons vegetable oil until cornstarch is dissolved. Mix in broccoli, snow peas, carrots, and green beans, tossing to lightly coat.
Heat remaining 2 tablespoons oil in a large skillet or wok over medium heat. Cook vegetables in oil for 2 minutes, stirring constantly to prevent burning. Stir in soy sauce and water. Mix in onion, salt, and remaining 1 teaspoon ginger. Cook until vegetables are tender but still crisp.
