Melt the butter in a medium saucepan over medium heat. Add the flour, whisk until well blended, and cook for 1 to 2 minutes, whisking often. Slowly pour in the chicken stock and mustard, whisking to mix. Bring the mixture just to a boil, then reduce to a simmer and cook until thickened, 5 to 7 minutes, whisking occasionally. Season the gravy to taste with salt and pepper. Store in the refrigerator for a few days, or freeze for 3 to 4 months.