I don't know if ours was perfect - but it was darned tasty. Beef is hard to come by here - so rib roast for Christmas was a special treat.
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Kacey, I agree with on the boughten au jus mixes - they just aren't good.
Here's how I do it. If you want you can skip most of this and just use some canned beef stock, but if you have time to make your own stock, the "jus" will be better.
Stock . . .
Take 4 or 5 pounds of beef bones cut into 2-3 inch long pieces (knuckle, shank, or rib bones) - clean most of the meat from them (save the meat scraps). Brush or rub the bones with a little canola oil. Roast them in a 400 degree oven for around 60 minutes. Take 3 or 4 crushed garlic cloves and the meat scraps you saved, mix then up with a little oil and throw them in the pan with the bones for the last 10 minutes of roasting.
Throw the bones in a stock pot, pick out the garlic cloves and throw the meat scraps in the pot - deglaze the roasting pan with a little cold water and pour it in the stock pot. Add enough cold water to the pot to cover the bones by an inch or so.
Bring the pot to a boil, then reduce the heat and simmer the bones for 2-3 hours - skim the surface regularly to remove the foam / scum. Keep the water level about the same as when you started.
After the simmer is done - remove the bones and strain the fluid. Chill the fluid, and then skim the surface film. At this point you have a good basic brown stock.**
Au Jus . . .
Put the stock in a pot and bring it to a boil - boil it until it's reduced by at least 50%.
When your rib roast has been removed from the roaster and set aside to rest, drain the grease from the pan. Be sure to save the meat juices though - set them aside.
Deglaze the pan - I like to use 1/4 cup of light brandy (wine works too - for those who don't cook with alcohol, water or lemon juice works too) .
Add a couple cups of the reduced stock to the deglazed roaster and bring it to a furious boil.
Now, depending on now you prepared the rib roast, the meat juices you saved may or may not be salty - taste them and get a feel for that. Begin adding them to the boiling stock in the roaster - if the juices aren't too salty add then all - if they are salty, just add enough to season the au jus to your taste. When it tastes right - remove it from the heat and it's ready to serve with the roast.
If the au jus gets a little thick, add some water.
Some folks add things like Worcestershire or soy sauce to it, some add powdered mustard or other seasonings or herbs - that's your call.
As I said you can use canned beef stock - it's definitely faster and simpler. I find that canned beef stock can taste a little harsh. One way to deal with that is to go with 3 parts beef and 1 part chicken stock, that seems to smooth the flavor out a bit.
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**You can also roast some aromatics with the bones (onions, carrots, etc) and then add them to the stock pot too - you can also add an herb sachet to the stock pot too during the boil. But if I'm making the stock to use for au jus, I like to keep it focused on the beef flavors.