Bring the stock to a boil. Meanwhile, melt the butter in a heavy saucepan. Whisk in the flour and cook for 1-2 minutes until the roux is foaming and straw-colored.
Remove the pan from the heat, allow to cool slightly and whisk in three-quarters of the stock. Bring the sauce to a boil, whisking constantly until it thickens. Season lightly as the flavor will beco