1. Heat the oil in a 4-quart saucepan over medium heat. Add the beef and cook until it's well browned, stirring often. Remove the beef from the saucepan. Pour off any fat.
2. Stir the sauce and onion in the saucepan and heat to a boil. Return the beef to the saucepan. Reduce the heat to low. Cover and cook for 35 minutes, stirring occasionally. Stir the vegetables in the saucepan. Cover and cook for 15 minutes or until they're tender. Serve over the mashed potatoes.
Tip: To make 4 cups mashed potatoes: place 2 pounds potatoes, peeled and cut into 1-inch pieces, in a 3-quart saucepan. Cover them with water. Heat to a boil over medium heat. Cover and cook for 10 minutes or until the potatoes are tender. Drain in a colander. Mash the potatoes with 3/4 cup milk and 2 tablespoons of butter.