Originally Posted by grandhill
I was hoping someone on this site had some experience with this. I am concerned that reducing the oven time by a third will ruin the roast. I am afraid of trying it for the first time on Christmas!
With some caveats and reservations, what you're proposing has some merits. I've cooked sous vide
for years, both at home and professionally. What you want to do is very similar. That said, I can tell you from professional and personal experience that a big special occasion dinner is a lousy time to try something new, especially something radically new.
I will now give you the best two words of advice I can possibly think of: Meat. Thermometer. Get a decent thermometer, ideally an electronic probe with a braided steel cable and a temperature alarm, and you will never overcook meat again. Get the probe into the thickest part of the roast and set the alarm for the temp you want and you're good. The main thing to bear in mind is that as the meat sits and rests, heat will equalize meaning it will move the hotter part outside into the cooler part in the middle. We call this carryover cooking, and how much rise you get in internal temp depends on the size of the chunk of meat and the temp you cook it at.
One of the most classic ways that restaurants do rib roast is to heat the oven to 500 degrees, toss the meat in for 30 minutes, then turn the oven off and leave the door shut. The meat is left in the oven for about 6 hours. This might sound crazy but it works. It's not how I do it, but it works.
The nice thing about a prime rib roast is that it's a whole muscle. The vast bulk of the bacteria are on the surface. Kill that with heat and you're pretty good unless you do something really stupid. The best way to do prime rib is at very low temp. You need high heat to get a good sear/crust, but the lower the temp the more evenly it will cook. If you take a cold hunk of meat and cook it in a 400 degree F oven until the middle is, say, 130 you will get a very, very done outside and a pretty rare inside. We call this a "bullseye" and it's not what most people call good eats.
I'd personally suggest a more traditional conservative method for your Xmas dinner. This isn't the meal you wanna risk screwing up. Save the Iron Chef stuff for another day.