Piedmont — specifically the Langhe and Monferrato areas. The dish appears in 18th century Piedmontese cookery books and reflects the region's access to Ligurian coast seafood (tuna and anchovies) via the salt trade routes.
Vitello tonnato is cold roasted or poached veal served with a smooth, pale sauce of tuna, capers, anchovies, and mayonnaise — a dish that sounds improbable (cold meat with tuna sauce?) and is one of the finest things in Italian cooking. The Piedmontese version (as opposed to later simplified versions) involves slowly poaching the veal in a court-bouillon with the tuna and aromatics, which then forms the base of the sauce. The sauce is silky, pale ivory, and has none of the fishiness one might expect — it is smooth, savoury, and slightly acidic.
Cold vitello tonnato has a flavour that defies simple description: the veal is mild and slightly creamy (the fat of the topside); the sauce is rich, savoury, and barely identifiable as tuna — it tastes like a sophisticated, meaty condiment. Capers provide the essential acid counterpoint. The combination of meat and fish is ancient in European cooking and vitello tonnato is its most elegant Italian expression.
The veal (topside or eye of round) is tied as a roast, then poached very slowly in court-bouillon with canned tuna, anchovy, capers, onion, celery, carrot, white wine, and wine vinegar for 1.5-2 hours until the meat is just cooked through and still slightly pink. The cooking liquid is strained and reserved. The tuna, anchovy, and capers from the court-bouillon are blended with hard-boiled egg yolks, olive oil, lemon, and enough cooking liquid to achieve a thick, smooth, pourable sauce. Slice the cold veal thin and arrange on a platter, cover generously with the sauce, and rest overnight.
The overnight rest is non-negotiable — the sauce is absorbed into the veal and the flavours unify. Serve at room temperature (15-18°C), not cold from the refrigerator. Garnish with capers and thin lemon slices — both are part of the flavour, not just decoration. The court-bouillon poaching technique produces a veal with a subtle tuna flavour absorbed into the meat itself — this is what distinguishes the classical version from the simplified one.
Using a modern simplified sauce of canned tuna + commercial mayo — acceptable but misses the depth of using the court-bouillon. Over-cooking the veal — it should be just-pink inside when sliced cold; overcooked veal is dry under the sauce. Serving immediately — vitello tonnato must rest overnight for the sauce to penetrate the meat and the flavours to integrate. Not slicing thin enough — the meat should be almost translucent, 2-3mm maximum.
Marcella Hazan, Essentials of Classic Italian Cooking; Pellegrino Artusi, La Scienza in Cucina