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Heat oven to 180C/160C fan/gas 4. Toss the bread with 1 tsp oil, season, then scatter over a baking sheet. Bake for 10-12 mins or until crisped and golden.
Rub 1 tsp oil, half the lemon zest, some salt and plenty of black pepper over the tuna steaks. Set aside.
Gently heat the remaining oil in a non-stick frying pan. Add the anchovies and garlic, and cook for 1 min, mashing the anchovy to create a purée. Scrape into a into a large bowl, then stir in the mayo, buttermilk, mustard, Worcestershire sauce and parmesan, plus the rest of the lemon zest and 1 tbsp lemon juice to make a loose dressing. Tumble the leaves into the dressing and massage well to make sure they are well coated. Leave for 5 mins so that the dressing begins to soften the cavolo nero.
Wipe out the frying pan and set over a high heat. Sear the tuna for 2-3 mins each side until just pink in the middle, or longer if you prefer. Let it rest on a plate for 1-2 mins. Pile the dressed cavolo nero onto plates, top with the fish, then scatter with the shaved parmesan and croutons. Drizzle with a little more extra virgin olive oil, if you like, then serve with the rest of the lemon, cut into wedges.