Melt butter in a medium saucepan over medium heat. Add the flour and cook, stirring, for 2 minutes. Whisk in the mustard and pepper, and continue to cook for 1 minute. Slowly whisk in the chicken broth and heavy cream. Bring mixture to a boil, whisking constantly. Reduce the heat to medium-low and simmer until the sauce is thickened and reduced to about 1½ cups, about 20 minutes. Yield: 1½ cups.