Lastly, you can use the same beurre manié technique that's used to thicken soups. Knead equal parts flour and softened butter into a ball, and pop that into the simmering gravy — that's all it ...
Everything took hours to make, but the runny gravy waters everything down as much as it adds flavor. A cooking slurry of water and either cornstarch or fluffed flour can thicken things up ...