When ready to assemble put a large pot with salty water on the fire and bring to a boil.
Cook the pasta sheets in it for 1 minute. Do this in batches (I use a shallow but large pot and I cook them in 1 layer, so I am sure they do not stick together).
Remove them with a slotted spoon and put them on a clean tea towel to cool down.
Mix the Mushrooms with the the Taleggio and béchamel sauce.
Now you are all set to start assembling your lasagne. Start with a layer of béchamel, Taleggio and Porcini sauce at the bottom of an oven proof dish. Add a layer of cooled pasta sheets, making sure not to overlap them. Now cover them with a layer of béchamel, Taleggio and Porcini sauce and grated Parmigiano Reggiano. Add another layer of pasta and repeat the layers until you run out of pasta.
Finish off with some béchamel, Taleggio and Porcini sauce and Parmigiano Reggiano.
Cook the lasagne in a pre heated oven at 180°C – 355°F for 20 minutes, then grill for 5 to 10 minutes until the top gets golden brown. Serve warm.