1. In 3-quart saucepan over high heat, in 1 inch boiling water, heat green beans and 1/2 tea spoon salt to boiling. Reduce heat to low; cover and simmer 10 minutes or until the beans are tender-crisp. Drain.
2. Meanwhile, cut beef top round steak into 1-inch chunks; set aside.
3. In small bowl, stir mustard, cornstarch, pepper, 1 cup water, and 1 teaspoon salt until blended; set aside. In 10-inch skillet over medium-low heat, in hot salad oil, cook onion until very tender, stirring occasionally. Increase heat to medium-high; add beef chunks; cook, stirring frequently, until beef chunks are browned on outside and delicate pink inside. Gradually stir mustard mixture into beef mixture in skillet; cook over medium heat, stirring constantly, until sauce thickens slightly. With slotted spoon, remove beef to medium-sized bowl, leaving sauce in skillet. Keep beef warm.
4. Reduce heat to low; add green beans to sauce remaining in skillet; heat through, tossing to coat beans. Arrange green beans on half of large platter. Line other half of platter with lettuce leaves; top with beef mixture.