Potato Gratin à la Normande – French Women Don't Get Fat

Ingredients

• 18 ounces potatoes (russet or Idaho), peeled and sliced thinly
• 1 ounce walnuts, chopped
• 2 tablespoons unsalted butter
• 3 Granny Smith apples, peeled, cored, and cut into 8 pieces each
• 1 Pont l’Évêque cheese
• 7 ounces sour cream or crème fraîche
• Salt and freshly ground pepper