Snagged and bagged right out of Vegetarian Cooking for Everyone (Hardcover) by Deborah Madison. Hubby made this lasagna once and received rave reviews for it.
This is incredibly time consuming, but worth it.
Since I am far too lazy to type it all out, here are the necessary parts I scanned and put up for links to be clicked. You can right click and save it.
Roux - http://i8.photobucket.com/albums/a34/illona_everlost/bechamel.jpg
Mushroom Lasagna - http://i8.photobucket.com/albums/a34/illona_everlost/mushroomlasagne.jpg
I've also put this up so if hubby ever wants to make it and he's not in the country and can't find the email...well, here it is.