1. Make cuts 1/8 inch apart across each potato, down to 1/4 inch from bottom (see notes).
2. 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.
3. Bake in a 425° oven until potatoes are tender when pierced, 40 to 50 minutes.