Whisk together flour and milk in a small bowl. Add to skillet. Bring
to boiling, stirring occasionally. Reduce heat to low; add salt and nutmeg
and simmer 5 minutes.
Heat oven to 400 degrees F. Lighly grease a 12 x 9 x 2-inch shallow baking
dish.
In same pot used to cook penne, toss penne with sauce, mushrooms, lemon
juice and pepper.
Combine all the cheddar, Swiss, and Monterey Jack cheeses in a bowl;
set aside 1/4 cup. Add remaining cheese to penne mixture, stirring gently.
Spoon into prepared baking dish. Sprinkle with reserved cheese.
Bake, uncovered, in heated 400-degree F. oven, 20 to 30 minutes or until
browned and bubbly.
Make-ahead tip: Prepare through all steps except baking. Refrigerate,
covered, up to 3 days or freeze up to 1 month. To serve, if frozen, thaw
in refrigerator overnight. Bake as directed.