Preheat the broiler. Transfer the duck to a large rimmed baking sheet. Strain the braising liquid into a saucepan, pressing on the garlic to mash it into the sauce. Bring the sauce to a simmer. In a bowl, blend the butter with the flour; whisk the paste into the sauce. Simmer, whisking, until thickened, 3 minutes. Season with salt and pepper and keep warm, whisking occasionally.