In a large pot, heat water and cream over medium heat until steaming. Stir in macaroni and 1/4 tsp (1 mL) salt. Cover, reduce heat to medium-low and simmer for about 8 min, stirring often, or until macaroni is almost tender.
Whisk flour, mustard and pepper into milk; stir into pot. Increase heat to medium; cook, stirring, for 3 to 5 min or until sauce is thickened.