How to Make It
Set a rack over a baking sheet. Rub 1/2 cup of the salt all over the roast and let stand for 10 minutes. Repeat with another 1/2 cup of the salt. Transfer the roast to the rack and refrigerate uncovered for 2 days. Bring to room temperature 3 hours before roasting.
Preheat the oven to 450°. Season the meat with pepper and roast for 20 minutes. Reduce the oven temperature to 225° and roast for about 1 hour and 30 minutes longer, until an instant-read thermometer inserted in the center of the roast registers 120°. Let the meat rest for 30 minutes.
In a bowl, mix the sour cream with the horseradish and the remaining 1 tablespoon of salt and pepper. Slice the roast and serve with the horseradish cream.