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Heat the oil in a large flameproof casserole over a medium heat and cook the onions for 8-10 mins until softened and starting to caramelise. Meanwhile, squeeze the sausagemeat from the skins.
Push the onions to one side of the casserole, then add the sausagemeat to the other. Squash it into smaller pieces using a wooden spoon, stirring occasionally for 10-12 mins until cooked and starting to brown in places. Mix the onions back in, then add the garlic, thyme, chilli flakes and tomato purée. Cook for another 1-2 mins. Tip in the plum tomatoes, crushing them with the back of the spoon. Season and bubble for 15-20 mins until the tomatoes have broken down and the sauce has reduced to a thick ragu.
Put the kettle on to boil. If using cavolo nero, remove and discard the tough stalks and roughly chop the leaves. For softer veg like spinach or chard, you can leave the stalks on. Put in a colander set over the sink and pour over a kettle of just-boiled water, then rinse the leaves under cold running water until cool enough to handle. Squeeze out as much water as you can, transfer to a board and finely chop. Stir the greens into the sausage ragu, cook for 1-2 mins until any excess liquid has evaporated (the mixture should be quite dry), remove from the heat and leave to cool a little.
Melt the butter in a separate saucepan over a medium heat. When sizzling, stir in the flour to make a sandy paste. Whisk in
the milk, a splash at a time, until completely incorporated. When the sauce is smooth and the consistency of custard, season well, grate in a good amount of nutmeg and stir in half the parmesan. Remove from the heat and set aside.
Drop the lasagne sheets into a large pan of boiling salted water one at a time to prevent them sticking together, then cook for 5-6 mins, stirring until soft enough to roll up but not fully cooked through. Drain and plunge into a bowl of cold water to stop the cooking process.
If the béchamel sauce has thickened as it’s cooled down, stir in a splash more milk. Spoon a third of the sauce over the base of a baking dish roughly 25 x 35cm. Lift a sheet of lasagne out of the bowl of cold water and lay on a board with one of the short ends facing you. Spoon a generous tbsp of the sausage ragu over one end, then roll it up to enclose the filling, making a short cannelloni – there will be a little overlapping pasta. Place it in the baking dish, then continue with the remaining ragu and lasagne sheets, arranging them in the baking dish in two rows of six cannelloni. Spoon any remaining ragu over the top, then pour over the béchamel sauce to cover all the cannelloni rolls. Sprinkle with the remaining parmesan. Will keep covered and chilled for up to two days or frozen for up to two months. Leave to cool completely first. Defrost thoroughly in the fridge overnight before cooking. Heat the oven to 200C/180C fan/gas 6, then bake the cannelloni for 40 mins until bubbling at the edges and golden brown on top. Grind over some black pepper, if you like, and serve with a crisp green salad.