I first made this in 1996 when it first appeared in Eating Well; I still have the clipped article and I've made it so many times the copy is stained. I have made a few alterations. I make the sauce in the microwave using a 4 cup Pyrex bowl. Heat all the milk until simmering (about 90 seconds) then whisk in flour, heat another minute or so until thickened, whisk smooth then proceed with the recipe as written. I use a lot more spaghetti sauce (homemade). I put 1 cup in the lasagna pan to begin and 2 cups in the mushroom mixture. I bake it at 350 for an hour then let it cool, refrigerate overnight, then reheat a day or so later. Gives the flavors time to blend. Frequently I make it in two 8 x 8 pans and freeze one after the initial baking.