HEAT oven to 375ºF.
MELT 1 Tbsp. butter in large skillet on medium-high heat. Add mushrooms; cook 6 min. or until tender, stirring frequently.
MELT 2 Tbsp. of the remaining butter in medium saucepan on medium heat. Whisk in flour; cook and stir 1 min. Gradually whisk in milk, then sour cream; cook 4 min or until thickened, stirring constantly. Stir in shredded cheese, salt and pepper; cook 1 min. or until melted.
GREASE 3-qt. casserole with remaining butter. Layer 1/3 each of the potatoes and mushrooms in casserole; repeat layers twice. Cover with cheese sauce. Sprinkle with Parmesan; cover.
BAKE 1-1/2 hours or until potatoes are tender and top is golden brown, uncovering after 1 hour. Let stand 10 min. before serving.