au gratin potatoes recipe

2 pounds potatoes, thinly sliced (Yukon Gold or Russet potatoes work well) - 2 cups heavy cream or half-and-half - 1 cup grated Gruyere cheese - 1 cup grated sharp cheddar cheese - 1 medium onion, thinly sliced - 2 cloves garlic, minced - 2 tablespoons butter - 2 tablespoons all-purpose flour - 1 teaspoon salt - 1/2 teaspoon black pepper - 1/4 teaspoon nutmeg (optional) - Fresh parsley, chopped (for garnish, optional)