SELECTING THE BEEF
Shopping for a roast can be confusing because the very same cut of meat goes by several different names. "Prime rib" is the most famous term, but the word "prime" actually describes the grade of the meat, not the cut. (The top three grades of beef are Prime, Choice, and Select.)
Meats graded "Prime" are sold almost exclusively to restaurants, so you probably won't find true "prime rib" at the grocery store. Instead, look for roasts labeled "rib roast," "eye of the rib roast" or "standing rib roast."
A boneless rib roast may be called "eye of the rib" roast -- or if the ribs are still attached, a "standing rib" roast. The meat will be more flavorful if you roast it with the ribs still attached, but a boneless roast is definitely easier to carve. If you buy a roast with the ribs attached, have the butcher remove the the backbone, or the roast will be difficult to carve.
HOW TO SEASON PRIME RIB
Most prime rib recipes call for very simple seasonings. Prime rib roast doesn't need a marinade or any complicated preparations; the meat speaks for itself. If you like, prepare a simple seasoning rub: fresh herbs, lemon zest, garlic, pepper and Dijon mustard are all excellent matches for prime rib. But don't salt the roast until right before cooking.
You can cover the meat with the spice rub up to 24 hours in advance; wrap it tightly with plastic wrap and refrigerate until you're ready to roast.
To infuse even more flavor, sliver the garlic, make tiny slits in the roast and insert the garlic bits.
HOW TO COOK PRIME RIB IN THE OVEN
No need to bring the meat up to room temperature first; you're going for a pink center, so it's okay if the outside heats up faster than the inside.
Place the meat in a roasting pan that's slightly bigger than the roast itself. If the pan is too big, the juices from the meat will spread out in the pan and evaporate. For a boneless roast, it's best to use a roasting rack.
If you've chosen a bone-in roast, the bones themselves will serve as your roasting rack. One side of the meat will have more fat on it; you want the fatty side facing up so the meat will baste itself as it cooks.
Don't add water to the pan, and don't cover it!
PRIME RIB COOKING TIME AND TEMPERATURE
How long does it take to cook a prime rib? Well, it depends. There are three ways you can roast a prime rib:
Low temperature for a long time. At 325° F (165° C), the meat will take about 17 to 20 minutes per pound.
High temperature for a shorter time. At 450° F (235° C) for the first 30 minutes and then reduce the temperature to 325° F (165° C), allow about 13 to 15 minutes per pound.
Both high and low temperatures. start high to sear the outside, then turn down the oven after 30 to 45 minutes to finish.
Your roast will shrink less if you cook it low and slow, but you won't get the same flavorful, well-browned exterior that a high roasting temperature gives you.
THE SECRET TO A PERFECT PRIME RIB
A thermometer is the absolute best way to guarantee the roast turns out exactly the way you want it. For an accurate reading, push the thermometer into the middle of the roast, making sure the tip is not touching fat or bone (or the pan).
Medium Rare = 130-140° F (55-60 C)
Medium = 145-155° F (63-68 C)
Remember that the roast's temperature will rise at least 5 degrees after you remove it from the oven.
Let the roast stand for 15 or 20 minutes before carving to let the juices return to the center.
With prime rib, it's easy to satisfy everyone's preference for doneness. The slices taken from the ends of the roast will be the most done, and the middle will be the least done.
Serve with pan drippings transformed into a simple Au Jus with some Horseradish Sauce on the side.