Step 1

Stir together cracked peppercorns, salt, and rosemary leaves in a small bowl. Rub mustard all over roast. Sprinkle roast evenly with peppercorn mixture, pressing to adhere. Place roast on a wire rack set inside a rimmed baking sheet. Refrigerate, uncovered, at least 12 hours or up to 24 hours.

Step 2

Preheat oven to 450°F. Arrange onion, celery, carrot, garlic, and rosemary sprigs in a 16- x 12-inch roasting pan. Place beef roast, fat side up, on top of onion mixture. Roast in preheated oven 15 minutes. Without opening oven, reduce oven temperature to 300°F. Continue roasting until a thermometer inserted in thickest portion of meat registers 115°F, 1 hour and 15 minutes to 1 hour and 25 minutes. Remove roasting pan from oven. Transfer roast to a cutting board, and let rest 20 minutes. (Internal temperature will continue to climb to 125°F.)

Step 3

Whisk together sour cream, horseradish, salt, and pepper in a bowl. Cover and refrigerate until ready to use.

Step 4

While roast rests, place roasting pan on stovetop over medium. Add butter to onion mixture in pan, and cook, stirring constantly, until melted. Sprinkle flour over onion mixture; cook, stirring constantly, 2 minutes. Gradually add stock, whisking constantly until flour mixture is smooth. Bring mixture to a simmer over medium; simmer, whisking often, until thickened, 3 to 4 minutes. Season gravy with additional salt to taste. Pour gravy through a fine wire-mesh strainer into a small bowl; discard solids, and transfer gravy to a gravy boat.

Step 5

Slice beef roast into 1/2-inch-thick slices, and transfer to a large platter. Serve with horseradish cream and gravy.