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Heat the oven to 200C/180C fan/gas 6. Remove the tough ribs from the cavolo nero and wash the leaves. Plunge them into boiling water and cook for 7 mins. Drain well, then set the leaves aside to cool.
Squeeze the excess water from the cavolo nero (you have to do this really well or you’ll have watery strudel), then roughly chop it and put in a bowl.
Heat the oil and 15g of the butter in a pan, then fry the onion for about 10 mins until soft and pale golden. Add this to the cavolo nero. Season.
Add a little more oil to the pan and cook the mushrooms over a high heat so they get a good colour. Cook for 10-15 mins until the water comes out of the mushrooms and has evaporated (if you don’t do this your strudel will be soggy). Season and add to the cavolo nero.
Cook the potatoes until tender, by either steaming, boiling or microwaving them, then cut into slices the thickness of a pound coin. Add the crème fraîche, parmesan, 100g of the fontina and the nutmeg to the cavolo nero, then add the potatoes and mushrooms and season. Stir everything well.
Melt the rest of the butter. Lay a sheet of filo on a large baking sheet. Brush lightly with melted butter, then put another sheet on top. Do this until you have 8 layers of filo.
With the long side of the baking sheet towards you, spoon the filling along the pastry, about 4cm in from the edge. Sprinkle the remaining cheese on top. Carefully roll the filo and the stuffing into a fat log shape and tuck the ends underneath to seal. If the package doesn’t seal well, then use more filo sheets – one sheet torn in half should do the job – to wrap around the ends. Filo is very forgiving so this won’t look bad.
Brush the strudel with more butter, scatter over the poppy seeds and bake for 25 mins until golden brown.