Roast Beef with Horseradish Cream

Preheat oven to 475 degrees F. With side of knife, gently smash 3 cloves garlic; discard peel. Into small bowl, crush remaining clove garlic with press. Cut 1 rosemary sprig into 1-inch pieces; set aside. Remove leaves from other sprig; discard stem. Finely chop leaves and add to bowl with crushed garlic along with 1 teaspoon oil, 1/4 teaspoon salt, and 1/4 teaspoon freshly ground black pepper; set aside. In 18" by 12" jelly-roll pan, combine potatoes, remaining 1 tablespoon oil, smashed garlic, snipped rosemary, 1/4 teaspoon salt, and 1/4 teaspoon freshly ground black pepper until well mixed. Spread in even layer, making space in center of pan for beef. Place beef in center of pan, fat side down; rub with reserved garlic-rosemary mixture. Roast 20 minutes or until beef browns. Reset oven control to 350 degrees F. Roast 8 to 10 minutes or until temperature on meat thermometer, inserted into thickest part of beef, reaches 130 degrees F; transfer to cutting board. Cover loosely; let stand 10 minutes. Transfer potatoes to platter. Meanwhile, whisk cream, horseradish, mustard, vinegar, 1/8 teaspoon salt, and 1/8 teaspoon freshly ground black pepper until well blended. Slice meat thinly; serve with potatoes and horseradish cream.