In-N-Out Burger is a wildly popular Southern California-based restaurant chain known for its juicy burgers, fresh-cut french fries, and superb milkshakes. Though the standard In-N-Out menu is rather simple, customers in the know are familiar with its "secret menu," which includes a special item called a mustard burger.

Nothing on In-N-Out's standard menu comes with mustard, so fans of this condiment need to request a mustard-grilled burger to get their fix. This sandwich is made by putting a beef patty on the griddle, then spreading mustard on top before flipping it so that it can seep in and imbue the browned meat with tangy flavor.

These mustard-grilled burgers also typically come with pickles, providing an extra sour kick and crunch. Another way to get a mustard-grilled patty is to order your burger in the notorious “animal style,” which comes with a mustard patty, pickles, grilled onions, and extra Thousand Island-style signature sauce.