Potato gratin with mushrooms

A tasty variation of scalloped potatoes.

1/4 cup butter, divided
1 lb. mixed fresh mushrooms (button, crimini), sliced
3 Tbsp. flour
1-1/2 cups milk
1/2 cup Sour Cream
1 pkg. (8 oz.) KRAFT Finely Shredded Italian* Five Cheese Blend
1/2 tsp. salt
1/4 tsp. pepper
2-1/2 lb. Yukon gold potatoes (about 8), peeled, sliced 1/8 inch thick. I used a salad shooter type to slice them nice and thin.

2 Tbsp. Grated Parmesan Cheese.

How to make it.....
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.

This was really a very tasty dish!