The reason I like cooking the roast so slow is that instead of the meat being very well done on the outside and real rare on the very center, the meat will have about 1/2" of well done on the outside and the rest will be evenly cooked.
Remember that Real Rare will be a little tough, so it is best served about half way between rare and med. rare
Again, a good older Cabernet goes GREAT with this.
Buying and trimming:
The following are ideas that I've read and re-written from some of my best web-sites.
When ordering the rib roast from a butcher, be sure to request a “choice” roast cut from the SMALL LOIN END; the best being ribs 12 through 10 (???). Have the butcher cut off the chine/backbone. The rib bones look best if they are shortened and frenched, have the butcher do this for you as well.
I read in another cookbook that the "small loin end" is the 1 - 3 ribs, so I would be careful what I ask for, as long as you say the small loin end.
To add more confusion to this, I've also had a "center cut". This cut is more meaty, with less fat inside, but it is also a little dryer tasting (because of less fat inside).
NOW, WHAT I'VE LEARNED:
Every once in a while, one of our local supermarkets has prime rib on sale. The last 2 times my buddy bought one, he (by accident) bought roasts from different ribs. The last time we had prime rib (pictured), it was the best and most tender. We both thought the same thing, and after we were finished eating, I looked it up on the "Bovine Mycology" web-site. What we ate that night comes from the #7 - #10 bones. I then asked my butcher at a different supermarket about the 7 - 10 bone, and he confirmed that this was the most tender part of the roast.
Now that I've totally confused you, enjoy Prime Rib, any way YOU like it.
1. Preheat the oven to 500.
2. When purchasing a roast, purchase one rib per person if you're having "MEAT DAY". If your serving it with sides, you should probably have about 3/4 pound per person. Be sure to ask your guests how big they want their piece when you cut it. I don't usually serve it with the bone (it takes up too much room on the plate.
3. Prepare the Roast by inserting slivers of Garlic all around, oil, then rub lots of Salt, Pepper, and Blacken into the meat, lay the sliced onions on top after you have placed it on a rack in a baking dish. Insert a meat thermometer into the thickest part of the meat in such a way that you will be able to see it through the window in the door without having to open it.
4. Place the meat in the oven for 20 minutes then turn down to 275 deg., without opening the door. Cook an additional 1 3/4 hours or until the meat thermometer reads 110 - 120 degrees, 110 degrees will give you a rare roast, pictured, and 120 degrees will end up med rare. If you like well done meat, don't waste your money on a prime rib, purchase a chuck or a top round and make a pot roast out of it.
Cooking time should not change because you have a larger piece of meat, because, although the meat will be longer, the diameter will usually remain about the same. This timing works good for a roast that is 6" - 7" in diameter.
5. This temperature (120 degrees) makes for a medium rare roast. Remember, don't cook it rare because it'll be quite tough.
6. Take the roast out of the oven and let it rest for 20-30 minutes loosely covered with alum. foil; this allows the roast to finish cooking and the juices to re-distribute. Don't worry about letting it rest too long because there isn't such a thing with Prime Rib (it's usually served at room temperature). Letting it rest, covered, lets it finish cooking. If you leave the thermometer in, you'll notice that it will actually go up about 10 degrees. If you wish to make gravy (sauce), you can set the meat on a separate platter, cover with aluminum foil, and use the pan drippings for the base of a sauce.
7. If you don't have a rack to place the meat on, you can use rough cut carrots and onions. Make sure you spray some Pam on the pan prior to putting the vegetables in. Later, you can place these vegetables into the pot you're making the gravy in, and grind them up with an immersion blender to become part of the gravy / sauce.