Venice — the castradina tradition dates to the earliest period of the Venetian Republic's trade connections with Dalmatia. The November 21 Festa della Salute (dedicated to the Madonna della Salute, commemorating the end of the plague of 1630-1631) is still the occasion for castradina in Venice.
Castradina (or castradina affumicata) is one of the most unusual preparations of the Venetian culinary calendar: smoked, salt-cured mutton (castrate — from castrated rams), imported from Dalmatia for centuries, traditionally prepared as a soup for the Festa della Madonna della Salute (November 21) — Venice's principal civic-religious festival. The smoked mutton is soaked for 24-48 hours to remove excess salt, then slowly braised with Savoy cabbage, onion, and wine until both the mutton and the cabbage are completely tender and the broth has taken on the smoke, salt, and fat of the mutton. It is a dish that exists only in Venice at this specific time of year.
Castradina soup is smoky, slightly salty, and deeply savoury — the long soaking reduces but does not eliminate the salt-smoke character, which perfumes the broth and the softened Savoy cabbage completely. The mutton itself is yielding and falls apart; the cabbage is the vehicle for the flavour. It is a dish of singular seasonality — November in Venice, autumn light on the lagoon.
Soak the salt-smoked mutton in cold water for 24-48 hours (changing water twice daily) to reduce the salt level to palateable. In a large pot, bring the mutton to a simmer in fresh water — change the water once during the initial blanching. Then braise with Savoy cabbage (cut into wedges), onion, and dry white wine in enough water to cover. Simmer very gently for 2.5-3 hours until the mutton is completely tender and falls from the bone. Skim the fat during cooking. The broth takes on the smoke and salt character of the mutton — this is the flavour. Season only at the end.
Castradina is available from the Dalmatian import specialty shops of Venice in the weeks before the Salute; outside Venice, it is virtually unobtainable. A homemade version using smoked lamb shoulder cured in salt for 2-3 days approximates the effect. The Savoy cabbage softened in the smoked mutton broth is arguably more interesting than the mutton itself — it absorbs the smoke completely.
Insufficient soaking — the castradina will be too salty without the full 48-hour soaking. Not skimming the fat — the smoked mutton renders significant fat; skimming during cooking prevents a greasy broth. Using the same water without changing — the salt removed from the mutton will be reabsorbed; fresh water at each change is essential.
Marcella Hazan, Essentials of Classic Italian Cooking; Slow Food Editore, Veneto in Cucina