secret menu; off-menu item
Refers to dishes or services that are not listed on the regular menu but can be ordered by those in the know. Often used in restaurants, bars, and other service establishments.
This restaurant has a secret menu that only regulars know about.
I heard the off-menu ramen is incredibly spicy.
Compound of 裏 (ura, 'back, reverse, hidden') and メニュー (menyū, 'menu'). The term likely arose in the context of restaurants and bars offering unlisted items to regular customers.