In-N-OutMustard Spread is applied to the burger patty before it's grilled upMustardBurgerAsk for your burger to be "Mustard Grilled" when ordering.All Locations4 - MediumFreehttp://hackthemenu.com/in-n-out/secret-menu/mustard-grilled-patty/Looking for a little extra zing? One of the more popular items on In-N-Out Burger's secret menu is the In-N-Out Mustard Grilled Patty . By requesting this item, the team over at In-N-Out Burger will slather the patty with mustard before grilling it. The result is a burger with a little more kick. Sounds good, right? If you want to go even further out of the box, we suggest ordering a " Protein Style Burger " with an In-N-Out Mustard Grilled Patty as the flavor just bursts through the lettuce wrap. Enjoy! Be sure to check out the In-N-Out Secret Menu as well as visiting our home page to check out the complete list of All the Secret Menus from all your favorite restaurants!