Place potato in a large saucepan; add water to cover. Bring to a boil; cover, reduce heat, and simmer 10 minutes or until tender. Drain potato, and place in a large bowl. Add milk and next 4 ingredients. Beat at medium speed of an electric mixer 2 minutes or until smooth. Set aside, and keep warm.
Cook green beans according to package directions, omitting salt; drain. Set aside, and keep warm.
Combine 2 tablespoons broth and cornstarch in a small bowl, stirring well. Set aside.
Coat a large nonstick skillet with cooking spray; add oil. Place over medium-high heat until hot. Add onion and next 4 ingredients; saute 5 minutes. Add cornstarch mixture and remaining broth to skillet, stirring constantly. Bring to a boil; reduce heat, and simmer, uncovered, 5 minutes or until thickened, stirring often. Add green beans and chicken; cook until heated.
Spoon chicken mixture evenly into 4 (2-cup) baking dishes. Top with mashed potato mixture, and place on a large baking sheet. Bake at 450° for 25 minutes or until bubbly and golden.