Place a medium pot over medium-high heat with the butter. Sprinkle the flour over the melted butter and cook for about 1 minute. Whisk the stock and cider into the mixture and bring up to a bubble. Add the mustard, if using, thyme and season with salt and pepper. Simmer until thickened, 4-5 minutes.
Serve warm over your favorite meat or stuffing and/or side dish.