This is mostly from Cook's Illustrated, combined with a few steps that I found online. When roasting at these exceptionally low temperatures, most recipes call for browning the exterior as a separate step, either in the oven for a few minutes at exceptionally high heat (before or after roasting), or on the stovetop in a large pan. I really don't bother. I found a recipe for Horseradish Cream Sauce that might go well with this.
Ingredients
1 3-rib standing rib roast (7 pounds) If aging, purchase a week in advance
Salt and ground black pepper
Instructions
1. If aging the prime rib, coat with kosher salt, place on a rack over a pan, and refrigerate uncovered for up to a week.
2. Remove from the refrigerator and set at room temperature for 3 hours before roasting.
3. Preheat oven to 200 degrees. Tie the roast with kitchen twine between the ribs. Coat generously with salt and pepper.
4. Roast until internal temperature registers 130 degrees for medium-rare, about 1/2 hour per pound. Let stand twenty minutes before carving (a bit longer is fine). Serve with Horseradish Cream Sauce.