In a large mixing bowl, combine garlic, pepper, cornflour and 1 teaspoon vegetable oil. Add beef and mix well to coat.
In a large wok, heat 2 tablespoons of oil over high heat for 1 minute. Add beef and cook, stirring, for about 2 minutes, or until it begins to brown. Transfer beef to a large bowl and set aside.
Wipe out wok with paper towel then heat remaining oil for 1 minute on high heat. Add onion and cook, stirring, until tender. Add green beans and stock and stir. Reduce heat to medium, cover and simmer for 4 to 5 minutes, or until beans are tender-crisp. Stir in soy sauce and beef. Cook, stirring constantly, for 1 or 2 minutes, or until heated through.