Bring a wide pan of water to the boil, add 1 teaspoon of sea salt and cook the lasagne sheets 2 at a time, they take just 1 minute. Remove with tongs, into a bowl of cold water. Repeat until all of the sheets are cooked. Remove from the bowl and place on a large clean tea-towel and lightly brush with olive oil.
To make the filling, melt the butter in a pan, add the onion and stir through, cook until soft and sticky. Stir in the garlic and cook for another minute. Add the mushrooms, seasoning, nutmeg and stir through, cook until any liquid from the mushrooms evaporates. Finely chop the porcini, chestnuts (if using) stir in and then add the wine and herbs. Once again cook until the liquid evaporates and the flavour is concentrated. Spread onto a dinner plate and leave to cool slightly.
Season the ricotta to taste with salt and pepper. Spread over the lasagne sheets, then add a layer of the spinach, sprinkle with a little seasoning and nutmeg. Top with the mushroom mixture and sprinkle with Parmesan.
Holding the edge of the tea-towel, roll up the lasagne into a long Swiss roll. Remove the tea-towel and cut the roll into 12 thick slices. Butter a casserole or gratin dish and spoon the white sauce into the base, top with the slices of rotollo so that the filling is facing upwards and is exposed. Dot the top all over with pieces of taleggio cheese and sprinkle with breadcrumbs. Bake in a preheated oven 190C/Fan170C/Gas 5 until golden and bubbling – about 35 minutes.