Ris de Veau aux Truffes

Soak the sweetbreads in cold water for 1-2 hours, changing the water once or twice; drain and rinse. Put them in a pot of cold salted water, bring to a boil and simmer for 5 minutes. Drain, rinse and trim the sweetbreads, discarding ducts and skin. Press them between two plates with a weight on top and leave until cold.

Melt half the butter in a saute pan, add the onion and cook over a