Which of the following items is commonly considered an extra on the menu?

The reason green chilies are considered an extra on the menu is that they are typically offered as an optional topping or ingredient that can be added to various menu items, enhancing flavor and customization for the customer. Extras like green chilies provide a way for customers to personalize their meals according to their taste preferences without being a standard component of the core items.

In contrast, the other choices represent items that usually constitute the main elements of a dish. Sourdough bread can be a standard part of certain sandwiches, while a chicken patty serves as the primary protein in items like a chicken burger. A fish sandwich similarly indicates that the fish itself is a significant component of the meal, not an additional extra. Thus, green chilies stand out as a customizable element that enhances the meal but isn't necessary for the main item itself.