Potato Gratin with Mixed Mushrooms

What You Need

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• 1/4 cup butter, divided
• 1 lb. (450 g) mixed fresh button and cremini mushrooms, sliced
  Giant Eagle — 2 for $5.00 until 06/28
• 3 Tbsp. flour
• 1-1/2 cups milk
• 1/2 cup sour cream
  Giant Eagle — 3 for $5.00 until 06/28
• 2 cups Cracker Barrel Shredded 4 Cheese Italiano Cheese
• 1/4 tsp. pepper
• 2-1/2 lb. (1.1 kg) Yukon gold potatoes (about 10), peeled, cut into 1/8-inch-thick slices
• 2 Tbsp. Kraft 100% Parmesan Grated Cheese