Cardoon au Gratin (cardon in French) is one of the most popular vegetable dish in Geneva, Switzerland, and in the neighboring Alpines areas of Savoie, as well as in Lyon. Several Provençal recipes also use cardoons. It is a traditional Christmas dinner dish that requires a very meticulous preparation.
It sort of belongs to the artichoke family, and grows in countries like Spain, Italy, and North Africa
The name is an alteration from French ''Chardon'', coming from a Latin word meaning thistle.
As Teatpuller said, you have to peel them carefully,then remove the remaining small strings by rubbing them in a piece of clean cloth. Then you have to cut them in 4 inch long segments that you put immediately in cold water containing lemon juice so that they do not get discolored ( turning grey-black), then you boil them gently in water (again containing lemon juice) for 30 to 45 minutes, and cover them, in a enameled cast iron or pyrex baking dish, with a bechamel sauce flavored with a touch of nutmeg. You then top the whole thing with grated Emmenthal cheese and put it in a 375 degrees oven for about 40 minutes. When the top turns "gratineed", serve very warm.