How to do au jus

I'm making a prime rib for dinner on Christmas day. As I found last year, making the au jus sauce from scratch is well worth the time and effort. The beefy depth of flavor you get is fantastic.

Here's the recipe. It's basically a beef stock made from oxtails that you make in a crock pot and then reduce down. Using the crock pot simplifies the process and helps extract lots of flavor and gelatin goodness.

4 lbs beef oxtails
1 small can tomato paste
2 large yellow onions
2 large carrots
1 bay leaf
1 teaspoon fresh thyme (or 1/2 tsp dried)
2 teaspoons salt

Preheat oven to 375
Season oxtails with salt and pepper. Rub each one with tomato paste and place on an oiled cooking sheet. Roast for 25 minutes. Turn. Roast for another 25 minutes.
Remove oxtails to large crock pot. (Leave fat on the pan.)

While oxtails are cooking, rough chop the onions and carrot. Toss veg with a couple tablespoons of olive oil and after removing the roasted oxtails, spread the veg out over the same cooking sheet you used to roast the oxtails.

Cook onions and carrots for about a half hour or until they start to caramelize and get some color on them. Add carrots, onions, bay leaf, thyme and salt to crock pot. Fill to top with cold water.

Place crockpot on low and cook for 12-24 hours. (I stick mine outside on a table outside so it doesn't overly perfume the house and drive the dog into a frenzy. The beefy oniony aroma is delicious, but gets old after about eight hours.)

Strain out solids and set aside. Cool down liquid and refrigerate so fat comes to top. Skim fat off. Reduce if desired and adjust seasoning as necessary.

But I need more stock!

I hear you. Fear not. All those solids have more to give. Take all the meat, bones, and vegetables you strained out and throw it all back into the crock pot again with fresh water up to the top and repeat the cooking process to yield another batch of stock. This one won't be as strong as the first, but can be used for soup or reduced down to a demi-glace.