1. First things first -- "au" is French for "with”. You don't make "au jus." You make jus (literally, juice). You serve the prime rib au jus, not "with au jus”. I'm not trying to be obnoxious or snobby -- lots of people call it that -- but it sounds odd to be making "with juice". Au jus is really just beef stock.
2. Even if you buy a boneless prime rib, I assure you that there were bones in it at some point. Make sure your butcher includes them with your purchase. (There shouldn’t be an additional charge.) You’ll use these bones to make the au jus. Or personally, if I was making in advance, I would buy a short rib & roast it as well. It's got a lot of cartilage which will make a rich, gelatinous stock. After braising, strain the sauce into a smaller saucepan & boil it down to reduce it somewhat. You don't want to reduce it to a thick sauce or glaze, as a jus should be "juicy."
3. AFTER it's reduced, add salt (boy, I've made the mistake of salting first & it just concentrates the salt in the remaining liquid - ugh), pepper, & if you like, soy or worcestershire sauce.
2IF MAKING HOMEMADE AU JUS - MAKE THIS FIRST - A DAY IN ADVANCE:
1. Toss oil through shallots together so that the olive oil coats everything well. Also coat the rib bones & any trim from the roast with olive oil.
2. Put them together in a roasting pan & roast in a preheated oven at 400 degrees F. for about 45 minutes turning every fifteen minutes or so. The bones should turn brown but not black. Adjust your oven accordingly.
3. From the oven put them in a large (16 qt) stock pot, fill with cold water to a couple of inches over the bones, then add thyme through peppercorns.
4. Deglaze the roasting pan by putting it on the stove top on low heat. Add a cup of red wine (can use water instead). Using a spatula scrape the scraps in the bottom of the pan into the wine & then pour it into the stock pot. Heat the stock to a good simmer (about 200 degrees F.) but not to a full boil. Cover it, put on the back burner for about the next 4 – 6 hours & let it simmer, periodically skimming the fat off of the top.
5. When you determine it is ready (at least the 4 hours) strain it through cheese cloth or a fine strainer into another pot, season with salt & pepper to taste. If it doesn’t have a beefy enough flavor you can finish it off with a little beef base (I recommend “Herb Ox Sodium Free Beef Bullion" - made by Hormel - but be sure to adjust your salt accordingly because bases are often salty.
6. Choice two (to buy a mix or au jus base) by the way can be enhanced by skipping the roasting process & just simmering everything & adding beef base to round out the flavor. While it is still good, the roasting of the bones releases the marrow which adds great flavor to your stock.