Carey Mulligan doesn't know what a cheesesteak is

Bradley Cooper might be the new cheesesteak king, but his "Master" co-star Carey Mulligan doesn't even know what that is. At the Dec. 12 premiere of "The Master, " Entertainment Tonight asked the film's director Cooper about opening a pop-up cheesesteak truck in New York just a few days ago. Mulligan, however, is a little confused about the exact premise of the Philly cheesesteak.

"Can I - what is a cheesesteak?" she asked. "Like steak? Steak with cheese?"

As a native of Philadelphia, Cooper gave her the recipe without judgment. "Well, it's like homemade bread No. 1," he replied. "You know, hollowed out, sliced ​​rib-eye steak, Cooper's cheese, caramelized onions."

Mulligan was naturally interested. "You really sold it to me," she replied. "Yeah. I made 600 of them the other day," he quipped.

Why didn't she eat the cheesesteak?

Carey Mulligan and Bradley Cooper attend the Netflix Masters Photocall in Los Angeles on December 12, 2023 . Axelle/Bauer-Griffin/FilmMagic/Getty Images

Understandably, Mulligan isn't entirely familiar with the art of cheesesteaks, as she's British and her director wasn't on the set of "The Master" to showcase his culinary skills. "No, I've heard about it, but I don't have physical proof yet," Mulligan said of the cheesesteak.

Cooper argued that he didn't make any food on the set because it wouldn't make sense for the biopic's subject, composer Leonard Bernstein. "No, Lenny didn't make a cheesesteak," he replied. "Lenny doesn't cook at all." "No, he doesn't," Mulligan agreed.

"He just smokes and waits to be fed."

Cooper's Food Truck

Bradley Cooper serves cheesesteaks on December 7, 2023 in New York, NY. MEGA/GC Photos/Getty Images

Cooper and Danny DiGiampietro, owner of Philadelphia's Angelo's Pizzeria, opened the pop-up food truck in New York's Greenwich Village, calling it "Danny & Coop's Cheesesteaks" and donating proceeds to Help Feeding New Yorkers in need.”

"We had just talked about opening a food truck and maybe a cheesesteak house in New York. So we decided to spend two days making cheesesteaks for people," Cooper told ET.

Celebrities who visited Cooper to support his new venture include Laura Dern, his ex-girlfriend Irina Shayk and rumored girlfriend Gigi Hadid. But he remains mum on what they think of his food. "I don't know, you'd have to ask them," he said, adding that cheesesteak orders are "very personal."