Stir together 2 teaspoons salt, garlic powder, pepper, thyme and fennel seeds in a small bowl. Put beef, rib side down, on a rack set in a roasting pan. Rub beef with spice mixture.
Place pan in preheated oven. Roast until a meat thermometer inserted into thickest part registers at least 52 C for medium-rare, 1 3/4 to 2 1/4 hours, or 60 C for medium, 2 1/4 to 2 3/4 hours. Transfer beef to a cutting board; let stand, covered with foil, for about 15 minutes (do not drain roasting tin).
Meanwhile, boil potatoes and onion in a pot of salted water for 10 minutes. Drain potatoes; sprinkle with remaining 1/2 teaspoon salt.
Remove rack from roasting pan, add vegetables and toss to coat with drippings. Return roasting tin to oven and roast until vegetables are golden and tender, 30 to 40 minutes.
Stir together soured cream, horseradish and chives in a bowl. Serve alongside beef and roasted vegetables.