Preheat oven to 350 degrees. Fry ground beef in vegetable oil.
Add onions and carrots. Cover and cook 5 minutes over medium heat. Blend in tomato paste, flour, and beef broth. Cook, stirring, 3 to 5 minutes. Stir in frozen vegetables and cook 10 minutes.
Cook and mash potatoes and spread over meat mixture in casserole dish. Dot with butter.
Bake uncovered at 350 degrees for 30 to 35 minutes.