In small bowl, whisk together beef stock, vinegar, cornstarch, paprika, thyme, Worcestershire sauce, rosemary, salt, pepper, and cayenne; set aside.
Cut onions in half lengthwise; slice lengthwise into thin strips. Seed and cut red and green peppers into thin strips. Set aside.
In large nonstick skillet, heat half of the oil over high heat; stir fry beef and garlic for about 2 minutes or until meat is browned but still pink inside. Transfer to a plate.
Add remaining oil to the skillet; cook onions and peppers, stirring often, for 4 minutes or just until starting to soften. Pour in sauce; cook, stirring, for 2 minutes or until thickened and glossy. Return beef and any juices to pan; stir until well coated and heated through.
Makes 4 servings