Notes: To make fan cuts, set equally thick wooden dowels or pencils on each side of each potato so you won't cut all the way through. Cut perpendicular to the dowels.
Make cuts 1/8 inch apart across each potato, down to 1/4 inch from bottom (see notes).
In a bowl or 9- by 13-inch baking pan, coat potatoes with oil and rosemary; sprinkle with salt and pepper. Place potatoes, cuts up, in pan or on rack beside beef.
Bake in a 425° oven until potatoes are tender when pierced, 40 to 50 minutes.