Preheat oven 375 degrees.
Bring milk to a simmer and set aside. Melt 8 Tablespoons butter. Add flour, cook 1 minute over low heat, stirring constantly w/wooden spoon. Pour hot milk into the butter/flour mixture all at once. Add 1 Tbsp. salt, add pepper and nutmeg, cook over med-low heat, stirring first with a wooden spoon, then a whisk for 3-5 minutes until thick. Set aside off the heat.
Slice mushrooms. Heat 2 Tbsp. oil and 2 Tbsp. butter. Add half of the mushrooms, sprinkle with salt, cook over medium heat for 5 minutes or until tender. (If your pan is big enough you can cook them all at once, otherwise, do half at a time).
In the bottom of 9 x 11 dish, spread some of the sauce, layer of noodles, more sauce, then 1/3 mushrooms and ¼ Parmesan. Repeat 2 more times.
Bake 45 minutes at 375 degrees.