au gratin potatoes recipe

4 large potatoes, thinly sliced - 2 cups heavy cream - 2 cups grated Gruyère or Cheddar cheese - 2 cloves garlic, minced - 1 teaspoon salt - 1/2 teaspoon black pepper - 1/4 teaspoon nutmeg (optional) - 2 tablespoons butter - Fresh parsley or chives, chopped (for garnish)