Poached Fresh Tuna Marinated with a Caper and Anchovy Sauce

Are you familiar with what is easily the greatest of all Italian cold dishes, vitello tonnato? It is composed of poached veal sliced quite thin and layered lasagne-style, each successive layer of veal covered with a sauce made of canned tuna (packed in olive oil), mayonnaise, anchovies, and capers. It must marinate at least 24 hours, during which time the flavors of tuna and veal interpenetrate, each dissolving into the other, and producing something that is like no other dish of mea