En vessie is a general term to refer to the French practice of cooking foods within a pig's sealed bladder. Poularde de Bresse Cuite en Vessie is, specifically a whole chicken prepared this way. It is a signature dish of Epicure, an elite restaurant in Paris that has earned a coveted 3 stars from Michelin.
One thing that sets the food at top restaurants apart from lesser establishments is how the food is presented.
(Or so I am told. I'm more of a Whataburger guy.)
When servers bring out the pig bladder chicken, they make a grand spectacle of cutting into the inflated bladder, which no doubt fills the air around the table with a savory bladder aroma.
-via Squirrel Matador