What key ingredient is present in Rob's crispy chicken sandwich?

Rob's crispy chicken sandwich features a key ingredient that enhances its flavor profile and complements the chicken's crispy texture. Mustard is known for its tangy and slightly spicy characteristics, which can add depth to the sandwich. In the context of a crispy chicken sandwich, mustard often serves as a condiment that not only elevates the taste but also adds a zesty kick that pairs well with the savory elements of the dish.

Other ingredients, while they may enhance a sandwich in various ways, don't traditionally define the flavor in the same way mustard does in a crispy chicken sandwich setup. Hence, mustard stands out as the essential ingredient that contributes to the overall taste experience of Rob's crispy chicken sandwich.