In a large bowl, blend cornflour, garlic, 1/2 of the ginger and 2 tablespoons vegetable oil until cornflour is dissolved. Mix in broccoli, mange tout, carrots and green beans, tossing to lightly coat.
Heat remaining 2 tablespoons oil in a large frying pan 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 ginger. Cook until vegetables are tender but still crisp.