The day before you plan to prepare the prime rib, place it on a baking sheet and salt all the meat surface with 4 teaspoons of salt, then refrigerate, uncovered, overnight.
The day of cooking, remove the beef from the refrigerator and bring to room temperature, 2 to 3 hours. Pat the beef dry with a paper towel.
Preheat oven to 475°F (245°C). Adjust the oven racks so the beef can roast in the middle of the oven.
In a small bowl, mix together the butter, garlic, and dried herb blend. Season with 2 teaspoons salt and 1 teaspoon pepper.
Slather a layer of about 1/3 of seasoned butter on the underside of the beef (the bone side). Place the beef in a heavy ovenproof skillet or roasting pan, bone side down. Spread another 1/3 of the remaining seasoned butter on the top and sides, reserving the remaining butter for the first baste.
Roast for 20 minutes. Reduce the oven temperature down to 275°F (135°C), spread the remaining seasoned butter over the top of the beef, then continue roasting, basting every 30 minutes with pan juices, until the internal temperature is 120°F (50°C) for medium-rare, about 90 minutes. (Start checking the internal temperature early in case it cooks more quickly.)
Transfer the beef to a serving platter, cover loosely with foil, and let it rest for 25 to 30 minutes; the internal temperature will rise to 125°F (52°C).
Meanwhile, make the horseradish crème: in a medium bowl. Combine the sour cream, horseradish, and lemon juice. Season with 1/2 teaspoon pepper.
Slice the beef and serve with the horseradish sauce.