
In-n-Out has a number of famous faces among its fans, including Sinners star Michael B. Jordan, who decided to celebrate his recent Oscars win by taking his golden statuette out for a burger - as you do.
Hilary Duff is also not shy about her love for the American fast food chain, having grown particularly fond of a special menu tweak that you may not even know about.
It's on In-N-Out's 'Not So Secret Menu' selection - advertised quite openly on its website - so it's more IYKYK than a truly guarded mystery.
But what it lacks in secrecy it more than makes up for in flavour, being a simple change that makes ALL the difference.
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So, how can you order it?
If you go on the brand’s site, you can choose six menu alterations, including a 4x4 burger with four beef patties, the 3x3 triple burger, the protein bun-free burger, a double meat, grilled cheese, or ‘Animal Style’ option.
Now, it’s the Animal Style that has everyone talking about, and that’s thanks to pop singer, Duff.
Speaking on the latest episode of Table Manners with Jessie and Lennie Ware, the songstress revealed why she always orders her beef burgers 'mustard grilled' and what it means.
Duff, 38, was asked what her ‘last supper’ would be, comprising of a breakfast, lunch, dinner, and dessert.
To this, she said if she were to eat her last supper, it would include her mustard grilled patty.
After asking if her menu picks would ‘have to go together’ and wondered if the hosts would be ‘disgusted’ with her random choices, she revealed her last meal would be an In-N-Out burger, as she asked: “How do you get better than an In-N-Out burger?”

The singer then said: “Have you ever asked for it mustard fried? They put mustard on the burger and then they fry it, and it just adds this depth.”
After salivating over her description, I headed to the burger company’s website to find out more about the mustard process, to find out that a mustard grilled/fried burger comes with caramelised chopped onions, an extra stack of pickle slices, tomato, and patties that are fried with mustard on the griddle.
Food blog writer and recipe recreator Serious Eats once tried to recreate the In-n-Out Animal Style mustard grilled burger, using frozen and shipped burgers for reference as the chain wasn’t local enough to visit.
They said: “The process is simple: Sear the patty on one side, and squirt some mustard on it as it sizzles. Flip the patty over so that the mustard cooks into the second side.
“The patties are covered with cheese, then caramelized onions are applied liberally to a single patty before topping it with the second, fusing all the elements together into a single cheesy, beefy, sweet, oniony, gooey, salty, oozy, crispy, meaty, savory, melty, delicious mess."
Oh, yes.