Things You'll Need
2 prime rib roasts
Kitchen twine (optional)
Large heavy skillet
Salt and pepper
Meat thermometers, or instant-read thermometer
Prime rib is one of the tenderest pieces of beef, for roasting. It is unusually well-marbled, making it especially juicy, as well. A large rib roast can serve a great many people because of its richness, but sometimes a single large roast is not available. That occasionally makes it necessary for a cook to roast two smaller pieces, in order to have enough to go around. Cooking two roasts at the same time is not difficult, though it does require some planning if they are not the same size.
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Staggered Cooking Times Technique
Inspect the roasts for any bone fragments or other debris left behind by the butcher. Wipe them away with a moistened paper towel, if found. Blot the surface of the roasts with paper towel, to dry them.
Check that the roasts are tied tightly, as this helps them cook more evenly. If not, retie them with kitchen twine. Simply wrap the twine around the roast and pull it tight, then tie it off with whatever knot is easiest for you. Tie the roast in three or four places, or as necessary to make a tight roll. It is best to tie the new twine before cutting away the old.
Weigh the roasts on a kitchen scale, if the weight was not marked on their packaging. Calculate cooking time for each roast at 20 to 25 minutes per pound. Make a note of the difference in cooking times. For example, if one weighs a pound more than the other, you will assume a 20 minute difference in cooking time, for the extra pound.
Heat a dry skillet over a hot burner, until it is on the verge of smoking. Sear the roasts on all sides to a rich, dark brown. Remove the roasts to a cutting board. Preheat your oven to 325 degrees Fahrenheit.
Rest the roasts, covered, for 30 to 60 minutes at room temperature. This step is optional, but results in a better finished product. The risk to food safety is minimized by searing the roasts, which kills the majority of microorganisms on the surface of the beef.
Season the roasts lightly with salt and pepper. Place the larger roast in your roasting pan, bone side down and fat side up. Place the roast in the oven, and make note of the time. When the difference in cooking time has elapsed, place the second roast in the pan alongside the first.
Roast until the internal temperature of each prime rib tests at 125 to 130 degrees Fahrenheit, for a roast that will be rare to medium-rare. Add another five to ten degrees for medium-rare to medium. Remove the beef to a serving tray, and cover loosely with foil. Rest the roasts for 20 minutes, to minimize the loss of juices when they are sliced. Serve hot.
Mated Roasts Technique
Select two roasts that are the same shape, and fit together well. If possible, two roasts cut from the same rack is best. Check and weigh the roasts as detailed previously. This time, add the weights of the two roasts and calculate an approximate cooking time from the combined total.
Sear the roasts according to the directions given previously, then remove them and set them onto a cutting board and let them rest for the same length of time. Season lightly with salt and pepper.
Position the roasts on your cutting board, so the ends match each other as closely as possible. Tie the two roasts together horizontally at top and bottom with kitchen twine. If the roast is large, a third tie around the middle will give a better result.
Transfer the tied roast carefully to the roasting pan, positioning it with the rib side down and the fat side up. Roast at 325 degrees until the internal temperature tests at 125 to 130 degrees Fahrenheit. Remove from the oven to a serving dish, cover loosely with foil, and rest for 20 minutes. Serve hot.
Many recipes call for the roast to be "oven-seared" by starting it at a high temperature for the first several minutes, then turning the oven down to finish cooking. This is impractical with staggered cooking times, so stove top searing is called for in this instance. Many chefs prefer this technique and use it for all roasts, as it yields a tenderer roast and does not cause as much shrinkage in cooking.
The beef must be dry when it is seared. If your recipe calls for the roast to be marinaded in advance, blot the marinade from the surface with paper towels before searing. More marinade may be brushed on, if desired, before roasting.
Add a dry rub or other seasonings, if desired, before roasting.
Clean and sanitize any surfaces that have come in contact with uncooked beef, and never allow cooked and uncooked beef to come in contact. Wrap and refrigerate any uneaten portions.