Prime rib is another name for a beef rib roast, which has the rib bones intact and is considered to be one of the best-quality beef cuts for roasting.
When purchasing, figure about 2-1/2 servings per 1 pound meat (about two servings per rib). Rib roasts vary in size from 4 to 10 pounds. Order your roast ahead from the butcher or meat department, especially during the holidays, when prime rib is especially popular. You may want to ask the butcher to remove the chine (back) bone for easier carving.
Tip: The name "prime rib" is a little misleading because it may not be prime, which refers to the highest USDA beef grade and has the most marbling. Choice, the most common grade found at market, has a little less marbling but should still be juicy and flavorful.
Readying your roast for cooking can be as simple as sprinkling it with salt and pepper or rubbing it with olive oil and a dry spice rub. Another easy way to add flavor is to stud the roast with garlic. Using the tip of a small knife, poke several 1-inch-deep slits randomly over the roast. Cut 2 or 3 garlic cloves into 6 to 8 lengthwise slivers, and tuck the slivers into the slits.
Preheat the oven to 350 degrees F. Place the roast, fat side up, in a 15-1/2x10-1/2x2-inch roasting pan. Insert an oven-going thermometer into the center of the roast, making sure it does not touch bone or the pan.
Tip: If you don't have an oven-going thermometer, use an instant-read thermometer inserted into the thickest portion of the roast to test the temperature (do not keep this kind of thermometer in the roast during cooking).
Roast the meat, uncovered, until desired doneness. For medium rare (135 degrees F), cook a 4- to 6-pound roast for 1-3/4 to 2-1/4 hours. For medium (150 degrees F), cook a 4- to 6-pound roast for 2-1/4 to 2-3/4 hours. After removing the roast from the oven, transfer it to a cutting board. Cover the meat with foil and let stand for 15 minutes. This allows the juices to redistribute, preventing them from draining out during carving. The meat temperature will rise about 10 degrees F while it stands (the timings and temperatures listed above allow for this).
Tip: Cooking prime rib on the bone helps to keep the meat moist.
On a cutting board, turn the roast on its side; remove a thin bottom slice if needed to stabilize the roast. Insert a large fork in the side of the roast, below the top rib. Carve across the front toward the rib bone. Cut along the rib bone with the tip of the knife to release the slice from the bone. Remove slice and transfer to a platter. Repeat with remaining slices.
Tip: Refrigerate leftover meat for up to 3 days. Use it to make warm or cold beef sandwiches on buns or dinner rolls spread with horseradish mayonnaise.