How long should I cook two 15 lb boneless prime ribs?

I got tasked with hosting our large Christmas party this upcoming Sunday. We have about 30-40 people coming, so we ordered 30 lbs of boneless prime rib which are split in two 15 lb pieces. I have never cooked prime rib before, let alone this much. I found cook time for a 15 lb roast (500 degrees for 15 minutes for a sear and 325 degrees for 3.5 hours). How much extra time should be added for two 15 lb prime ribs?



HalfPint December 16, 2020
If you don't already have one, get a good instant read thermometer. ThermoWorks Thermo Pro is $34. Even better would be an oven-safe probe (or 2, one for each piece of meat) with a digital display that can sit on the counter top.

My recommendation, given the amount of meat, is to increase the initial sear time (at 500F) to 20-25 minutes or until you get enough exterior browning that you like. In other words, the meat should not be a pasty gray.

Then proceed at 325F. Take the temperature of each after 90 minutes. To see where the internal temperature is. You can estimate how fast the meat is cooking. Then take the temp at 30 minute intervals until you get to about 10degrees from your desired doneness. Here's where the probe thermometer can be handy: insert probe into center of the each piece at the beginning of the sear. You can see how fast the temperature is increasing and know the exact temperature to take out the roasts. No need to open the oven to take the temp.

I recently cooked (separately) 2 rib roasts using the ThermoWorks probe. My 5lb rib roasts took about 90 minutes to cook to medium rare. I took out the roasts when the internal temperature was 136F. During the resting period, carryover heat raised that internal temperature to 145F. It was perfect, especially the second one which my husband declared was the best he's ever eaten. I can't see myself making prime rib any other way now.

It's much more accurate to take the internal temperature than rely on length of cooking time. 30lbs of rib roast is too precious to be taking guesses.
tcdclarke December 16, 2020
The only problem is, I have guests coming over at a specific time. If the internal temp gets to where I want before dinner time what do I do? If it isn't ready in time, I have to make them wait.
HalfPint December 16, 2020
If the roasts are ready before dinner time, they can sit and wait for dinner time without much hard. Roasts would have to rest for at least 20 minutes before you carve in any case. A little longer wait is not going to hurt a roast. Restaurants have prime rib cooked and ready prior to any order, so you should be fine if your roasts are done early.
HalfPint December 16, 2020
I would add an hour to 90 minutes extra prep time , to make sure that the roasts are given enough time to cook and rest before serving. Good luck!
HalfPint December 16, 2020
Clarification, I would start cooking about 60-90 minutes earlier than the estimated 3.5 hours for roasting.
Recommended by Food52