1. In a small bowl, mix together the oil, herbs, horseradish and salt & pepper. I left the amounts above not precise, as the amounts are really up to you, and how spicy you like your meat. My husband adds a lot, as we love the flavours, and also because, a lot of the rub falls off into the drip pan.
2. Rub the spice mix over the entire surface of the prime rib, then wrap meat in plastic, and refrigerate for at least 4 hours, or even overnight.
3. Bring your meat out prior to cooking, and allow time to have it come up to room temperature prior to putting it on the barbecue.
4. Preheat your barbecue on medium. Then unwrap the meat and place it in the centre of your oiled grill (with the centre burner turned off) or, put it on a rotisserie. We prefer to do ours on the rotisserie.
5. If you are doing your meat on a rotisserie, make sure to have a drip pan under your meat, it catches all the good drippings, and you can make gravy or a sauce with it (just make sure to strain it first). We put red wine or Guinness beer in the pan during the cooking process.
6. It will take about 2 to 3 hours, depending on how you like it done. We like it medium-rare, so we usually do it for about 2 1/2 hours. And this all depends on how big your roast is.
7. Remove your roast when it is about 5 degrees below the internal temperature you want, and let it sit, covered with foil, for about 15 minutes.

Here is a temperature guide for doneness:

1. Rare: 140 degrees F
2. Medium: 160 degrees F
3. Well Done: 170 degrees F