Pour the stock into a saucepan, bring to the boil and tip the dried mushrooms into a heatproof bowl or jug. Pour over the hot stock and leave to soak until it has been absorbed and the mushrooms have cooled. Drain the mushrooms over a bowl, pressing down on them to release all the stock. Reserve the stock, roughly chop the mushrooms and set aside.
Heat the olive oil and two-thirds of the butter in a large, shallow saucepan. Add the soaked mushrooms and bay leaf, and sizzle for a few mins. Turn up the heat, add the fresh mushrooms and garlic, and fry until soft. Season, scatter the flour over the mushrooms and cook until you have a thick, sticky paste. Pour in the mushroom stock and boil until everything is gloopy. Gradually add the hot milk, stirring between each addition, until you have a thick mushroom sauce, then simmer for 10 mins, stirring occasionally. Remove from the heat, fish out the bay leaf, then stir in the cream, prosciutto and parsley, and leave to cool slightly.
If cooking straight away, heat oven to 200C/180C fan/gas 6. Grease a rectangular casserole dish (about 22 x 30cm) with the remaining butter and cover the bottom with a layer of lasagne sheets. Using a slotted spoon, layer about a third of the mushroom mixture and some of the sauce over the lasagne sheets, then scatter over a little grated Parmesan and drizzle with a few drops of truffle oil, if using. Repeat until you have three layers of mushrooms (save some sauce in the pan), then top with a final layer of pasta. Now spread over the remaining sauce – it should be reasonably mushroom free. Scatter over the remaining grated Parmesan and drizzle with more truffle oil, if using. Can be made a day ahead, covered and chilled.
Bake the lasagne for 20-25 mins until golden and bubbling. Turn the oven up at the end of cooking if it’s not brown enough. Remove from the oven and leave for a few mins to stand. Cut into squares, scatter over the Parmesan shavings and drizzle over some more truffle oil just before serving, if you like.