Cook onions and carrots in salted and boiling water; drain. Season beef and form into 8 balls. Brown in fat placed in casserole. To make gravy add flour to drippings; brown. Add 1 1/2 cups liquid, using any left from cooking vegetables. Stir until thickened. Add to the meatballs the gravy, onions and carrots. Cover with the mashed potatoes. Bake in a moderate oven, 350ºF. until brown, about 20 minutes.