Prime Rib in a Roaster Oven

Nesco publishes a prime rib instruction for the 18-quart roaster oven, and it is the figure this page works from rather than a number from anywhere else. For a 6 to 8 pound beef rib roast:

• Combine the seasonings — the published version uses six cloves of minced garlic, a teaspoon of cracked black pepper and a teaspoon and a half of dried thyme — and press them evenly into the surface of the roast.
• Heat the roaster oven to 325F.
• Place the roast in the cookwell.
• Slow cook 13 to 15 minutes per pound.
• Finish by temperature, not by the clock: 130F rare, 135F medium rare, 140F medium, 150F well done.

At the published serving size that is 8 to 10 people from a 6 to 8 pound roast. The timing works out at roughly 80 to 120 minutes for a 6-pound roast and 105 to 160 for an 8-pound one, which is a wide enough band that the thermometer is not optional.

Treat every row as a planning estimate and the probe as the decision. A roast comes out when it hits its number, not when the timer says so.

This is the one thing a roaster oven will not do for you. It is a covered appliance; moisture leaving the meat stays in the box; and browning needs a dry surface and high heat. Left to itself, a rib roast out of a roaster is beautifully cooked and pale.

Three ways round it, in order of how well they work.

Sear first, in a pan. Get a heavy skillet genuinely hot, oil the roast rather than the pan, and brown every face including the ends — four or five minutes a side. Then into the roaster at 325F. This is the most reliable method and the easiest to time, because the roast comes out of the roaster ready to rest.

Finish in a hot oven. Pull the roast 5F to 10F short, move it to a preheated 450F to 500F oven for eight to twelve minutes to colour the outside, then rest. This gives the best crust but it needs the oven free at exactly the wrong moment on a holiday afternoon.

Lid off for the last stretch. Take the roaster lid off for the final twenty minutes and turn it up. It helps a little. It is not a substitute for either method above, and it costs you temperature control at the point where precision matters most.

Whichever you use, dry the surface hard with paper towel before the seasoning goes on, and salt the roast several hours ahead if you can — a dry surface browns and a wet one steams. The same logic drives dry brining a turkey.

Use a rack. A rib roast renders a lot of fat and a roast sitting in it braises on the underside while the top roasts. A rack lifts it clear, lets heat circulate underneath, and keeps the drippings usable for a jus rather than boiling the base of the meat.

Bone side down. If the roast is bone-in, the bones act as their own rack and insulate the meat above them. Fat cap up, so it bastes the roast as it renders.

Rest it properly. Twenty to thirty minutes, loosely tented with foil, somewhere warm but not hot. This is not optional with a large roast — carve it straight out of the roaster and the juice ends up on the board. The internal temperature will also carry on climbing several degrees while it rests, which is exactly why you pull it at the target number rather than past it.

Carve across the grain, thin. An electric carving knife earns its keep here more than on a turkey — a long even stroke through a rib roast is hard to do by hand without sawing. A board with a juice well is the other half of it; see best carving boards.

A rib roast is a finished-to-temperature cut, which means it does not tolerate holding the way a braise does. Keeping a medium-rare roast warm for two hours cooks it to medium. Plan to serve it within about forty minutes of it coming out, and use the rest period as the buffer.

What you can do is free the roaster the moment the roast is out. Wipe the cookwell, add an inch of hot water, drop it to its lowest setting and it becomes the holding oven for everything else while you carve — the method is in using a roaster oven to keep food warm.

Do not cook the sides in the cookwell alongside the roast. There is room, and it is still a bad idea: anything you add releases moisture into a box you are already fighting for dryness, and the roast will be paler for it. Sides go in the oven, which is now free.

If you are cooking prime rib because the turkey is going elsewhere, the same roaster handles a ham at 325F and 15 to 20 minutes per pound to 160F — roaster ovens for a ham — and the full set of times and temperatures across cuts is in roaster oven cooking times.

Cooking to the clock. Thirteen to fifteen minutes per pound is a planning figure covering a two-pound-wide band of roast sizes and a range of starting temperatures. The only thing that decides doneness is the probe.

Starting from refrigerator cold. A large roast straight out of the fridge cooks unevenly — the outside overshoots while the centre catches up. An hour on the counter before it goes in evens it out considerably.

Opening the lid repeatedly. A roaster has no window and recovers slowly. Every look costs you heat and extends the cook. Use a leave-in probe and keep the lid on.

Expecting a crust that was never coming. The single most common disappointment with a roaster-oven rib roast, and it is entirely avoidable by searing first. Decide which browning method you are using before the roast goes in, not when it comes out.