Lost brioche (also known as brioche perdue or brioche perdu) is a French dessert that derives from French toast (known as pain perdu in French), where the leftover brioche bread from a bakery was used to make both dishes. The main difference between brioche perdue and regular French toast is in its consistency, which is more gooey but decadent and sweet.
The picture above came from a restaurant I visited with my family at Christmas in Nice, France last year. It was my first time eating the dessert and it was so good, I went back again a few days later to have seconds! We initially didn’t know what the name meant and thought it undersold the dessert but now I know better!
In terms of a recipe, I didn’t ask at the restaurant (it never crossed my mind to make one) and it’s hard to find one online that isn’t just for French toast which is different. I did find one on Yummly which you can follow but it lacks ingredient quantities which you’d need. If anyone reading this has a recipe, I’d be happy to host it here.