Remove chicken neck and giblets; reserve for other uses. Pull off and discard lumps of fat. Rinse chicken inside and out, then pat dry. Brush skin with butter. Place chicken, breast up, on a rack in a 12- by 15-inch roasting pan. Roast, uncovered, in a 375 degree oven until a meat thermometer inserted in thickest part of thigh (not touching bone) registers 185 degrees, or until meat at thighbone is no longer pink (cut to test), about 1 hour. Using poultry shears or a knife, cut chicken into quarters. Arrange pieces, skin side up and slightly apart, in a shallow ovenproof serving dish (use a 9-by 13-inch rectangular or 12- to 15-inch oval dish). Sprinkle with 3/4 cup of the cheese. Return to oven and turn off heat. Skim and discard fat from pan drippings. Stir in mustard and 3/4 cup of the cream; bring to a boil over high heat and boil, stirring, until shiny bubbles form 3 to 4 minutes. Remove from heat, add remaining 1/2 cup of cheese, and stir until cheese is melted and sauce is smooth (if necessary, thin sauce with a little more cream). Keep sauce warm. Broil chicken 4 to 5 inches below heat until cheese is bubbly, about 1 minute. Pour sauce around chicken; garnish with watercress.