In a large bowl, blend cornflour, garlic, 1/2 of the ginger and 2 tablespoons vegetable oil until cornflour is dissolved. Mix in broccoli,snow peas, carrots and green beans, tossing to lightly coat.
Heat remaining 2 tablespoons oil in a large deep frying pan or wok over medium-high heat. Stir fry vegetables for 2 minutes. Stir in soya sauce and water. Mix in onion, salt and remaining ginger. Cook until vegetables are tender but still crisp.