Preparing the Roast:

• Allow the beef to stand at room temperature for 1 hour.
• Preheat the oven to 350°F.
• In a small bowl, stir together 2 teaspoons of salt, garlic powder, black pepper, thyme, and crushed fennel seeds.
• Place the beef on a rack in a roasting pan, rib side down.
• Rub the beef with the spice mixture.
• Roast in the oven until an instant-read thermometer inserted into the thickest part registers at least 135°F for medium-rare (about 1¾ to 2¼ hours) or 150°F for medium (about 2¼ to 2¾ hours).
• After roasting, transfer the beef to a cutting board and let it stand, covered with foil, for about 15 minutes.
• Do not drain the pan.
• Boil the halved potatoes and onion in salted water for 10 minutes, then drain.
• Sprinkle them with the remaining ½ teaspoon of salt.
• Remove the rack from the roasting pan, add the vegetables, and toss them to coat with the drippings.
• Return the pan to the oven and roast until the vegetables are browned and tender, which should take about 30 to 40 minutes.
• Making the Dipping Sauce:
• Stir together the sour cream, horseradish, and chives in a bowl.