Peppercorn Roast Beef with Horseradish Cream

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.

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.)

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

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.

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