Step 1

Preheat oven to 450 degrees. Remove roast from refrigerator. Whisk oil, flour, 4 teaspoons salt, sugar, and pepper in a small bowl.

Step 2

Boil potatoes in a large pot of salted water until fork tender, about 10 minutes. Drain. Peel, and halve each potato crosswise. Run tines of fork over rounded side of each half.

Step 3

Place roast in a roasting pan, ribs side down. Lightly score fat. Rub roast with flour mixture. Roast for 15 minutes. Reduce heat to 375 degrees, and continue to roast, basting frequently with pan juices, for 1 hour more. Add potatoes, tossing to coat. Roast, flipping potatoes and basting meat occasionally, until thickest part registers 135 to 140 degrees on an instant-read thermometer (avoiding the bone) for medium-rare, about 45 minutes more. (Total cooking time should be about 2 hours.) Transfer to a cutting board, reserving pan drippings for Yorkshire pudding; tent with foil, and let rest for at least 20 minutes (or up to 1 hour) before carving. To carve, cut between each rib, then cut each slice into desired portions. Serve with horseradish cream.