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Heat the oven to 200C/180C fan/gas 6. Halve any fatter artichokes lengthways and put all of them in a roasting tin in which they can lie in a single layer. Season and toss with half the olive oil. Turn everything over with your hands and roast for about 30 mins.
Toss the leeks in the rest of the oil and season. Add to the tin and cook for a final 15-20 mins. The leeks should be tender and the artichokes completely soft.
Mix together the crème fraîche and the buttermilk or milk. Set aside.
To make the dressing, beat all the ingredients together with a fork. Season. Taste, as you might want to adjust the sweetness or acidity.
Toss the Treviso lettuce or chicory with the dressing. Put the roast vegetables on a serving platter or shallow dish, scatter the Treviso or chicory over, then spoon on the crème fraîche mixture (or serve it on the side). Scatter over the hazelnuts and cheese, and serve.