Read this: A second New Haven gastronomical claim to fame is Louis' Lunch, which is located in a small brick building on Crown Street and has been serving fast food since 1895. Though fiercely debated, the restaurant's founder Louis Lassen is credited by the Library of Congress with inventing the hamburger and steak sandwich. Louis' Lunch broils hamburgers, steak sandwiches and hot dogs vertically in original antique 1898 cast iron stoves using gridirons, patented by local resident Luigi Pieragostini in 1939, that hold the meat in place while it cooks.

Louis' Lunch is popular for serving what?
What is the answer? (If it cannot be answered, return "unanswerable")
the hamburger