Place first 4 ingredients in a saucepan; bring to a boil, reduce heat, and cook 4 minutes. Add green beans; cook 5 minutes. Coat a nonstick skillet with cooking spray; add oil. Place over medium-high heat until hot. Add onion; saute 5 minutes. Sprinkle flour over onion; stir. Add broth mixture to onion; stir. Add Worcestershire sauce, tomato paste, and pepper; cook over medium heat until thickened, stirring frequently. Add Savory Pot Roast; cook 2 minutes.
Cook potato in boiling water to cover 10 minutes or until tender. Drain; mash. Add milk; beat at medium speed of an electric mixer until smooth. Stir in sour cream, salt, and 1 teaspoon chives. Spoon beef mixture into 2 (1 1/2-cup) casseroles coated with cooking spray. Spread potato topping over beef mixture; crisscross with tines of fork. Bake, uncovered, at 350° for 20 minutes. Sprinkle with cheese and remaining chives; bake 5 minutes.