In a small bowl, blend 2/3 cup (150 ml) of the broth with hoisin sauce and rice vinegar. Heat wok, then add 1 teaspoon (5 ml) oil plus beef. Stir-fry until browned, 1 to 2 minutes. Transfer meat to plate and drain fat from pan.
Heat remaining oil, then add onion and garlic and stir-fry for a minute. Add sweet potatoes, pepper and broth mixture. Cover and reduce heat. Simmer until sweet potatoes are tender, 10 minutes.
Mix remaining broth with cornstarch; add to pan. Boil for a minute. Return beef to pan; serve over rice.
Per serving (excluding rice): 328 calories, 21 g protein, 16 g fat (6 g saturated fat), 23 g carbohydrates, 3 g fibre, 49 mg cholesterol, 343 mg sodium