swiss chard and sweet potato gratin

INGREDIENTS

• 1/4 cup (1/2 stick or 2 ounces) butter
• 1 small onion, finely chopped
• 3 pounds Swiss chard, leaves and stems separated and both cut into 1-inch pieces
• Pinch of freshly grated nutmeg
• 2 cups heavy cream or whole milk
• 2 garlic cloves, minced
• 2 tablespoons flour
• 2 pounds medium red-skinned sweet potatoes (yams), peeled and cut into 1/8-inch thick rounds
• 1 tablespoon minced fresh Italian parsley
• 1 tablespoon minced fresh thyme
• Fine sea salt
• Freshly ground black pepper
• 1 1/4 cups (about 5 ounces) coarsely grated Gruyére cheese