Heat oven to 375°F. Spray 11x7-inch (2-quart) glass baking dish with cooking spray.
In 12-inch skillet, heat oil over medium- high heat. Cook onion in oil 5 minutes, stirring occasionally, until soft. Add beef; cook 5 to 7 minutes, stirring occasionally, until thoroughly cooked. Drain.
Add mixed vegetables, tomato paste, salt and pepper; cook over medium heat 5 minutes, stirring frequently, until vegetables are hot. In small bowl, mix broth and flour with whisk. Add broth mixture to beef mixture. Heat to boiling; cook 3 minutes, stirring constantly, until thick. Spoon beef mixture into baking dish. Spread mashed potatoes over beef mixture; fluff with fork.
Bake 20 minutes or until golden brown.