Inspired by this article in the LA Times, describing the Croque Matthieu from Pasjoli restaurant in Santa Monica. This description lured me in:
QuoteDuring a recent visit, my party fell silent as we each bit into a square of the sandwich. The bread, annealed and shiny, was all crunch and butter. Then there was crispy Gruyère cheese, like a nutty, sharp cracker cemented to the inside of the sandwich. It shattered into a puddle of rich, creamy Mornay sauce that melted into sweet caramelized onions and shaved Bayonne ham.
The decadence of French onion soup, the simplicity of a good jambon beurre and the thrill of a gooey grilled cheese were all present, crammed into this four-bite sandwich. Its unabashed decadence something to marvel.
Since the article also provides a bit more info on how it's constructed, I had to give it a try. Sorry for the bad photo.
But not disappointed with the result!