1. Heat the butter and sunflower oil in a large skillet. Add the onion and beef. Brown the beef on all sides. Transfer the beef and onion to a slow cooker.
2. Arrange the carrots, potatoes and bay leaves around the beef. Sprinkle with salt. In a heatproof bowl or measuring cup, combine the soup mix with 3 cups boiling water, stirring until smooth.
3. Pour the soup mixture over the beef and vegetables in the slow cooker. Cover and cook on HIGH until the meat is very tender, about 3 1/2 hours.
4. In a medium saucepan, combine the cornstarch and about 1 teaspoon water. Stir to form a paste. Add 2 cups hot liquid from the slow cooker and bring to a boil, stirring, until the gravy has thickened. Slice the beef and serve with vegetables and gravy.
