The idea is simple. In place of meat ragù, use a chunky-ish sauce made of finely chopped mushrooms (champignons and dried/soaked porcini; cook for about ten minutes in olive oil; salt, pepper, chopped fresh parsley; a scoop or two of your béchamel). You can layer in extra slices of mushrooms here and there as you construct the lasagne. Bake at 160 °C / 320 °F for about 45 minutes or until very golden and bubbly. Let sit for at least 20 minutes before serving. Everything else is the same as with making a traditional pan of lasagne, nicely outlined here (in Italian).