au gratin potatoes recipe

2 pounds russet potatoes, peeled and thinly sliced - 1 clove garlic, minced - 2 cups heavy cream - 1 cup grated Gruyère cheese - 1/2 cup grated Parmesan cheese - 1 teaspoon salt - 1/2 teaspoon black pepper - 1/4 teaspoon ground nutmeg - 2 tablespoons unsalted butter, cut into small pieces - Fresh thyme leaves, for garnish (optional)