Sunday Rib Roast

INGREDIENTS

• 1 1/2 tablespoons whole-grain mustard
• 1 (3-rib) standing rib roast (7 to 8 pounds)
• 1 1/2 cups good mayonnaise
• 3 tablespoons Dijon mustard
• 1 tablespoon prepared horseradish
• 1 1/2 teaspoons freshly ground black pepper
• 1/3 cup sour cream
• Mustard Horseradish Sauce, recipe follows
• 1 tablespoon kosher salt
• 1/4 teaspoon kosher salt