1Pat the beef dry, season with 2 teaspoons of salt and 1/4 teaspoon pepper, and let sit at room temperature for 1 to 2 hours.
2Peel carrots and parsnips, halve crosswise, and then halve or quarter lengthwise.
3Preheat oven to 375ºF.
4Combine grated garlic, chopped rosemary, and 1 tablespoon olive oil; rub all over the beef.
5Place beef in a roasting pan or rimmed baking sheet fat-side up, scatter carrots and parsnips around it, drizzle with remaining 2 tablespoons olive oil, remaining 1/2 teaspoon salt, and black pepper, and stir vegetables to coat.
6Roast for 30 minutes, then reduce oven temperature to 275ºF and roast for another hour until beef registers 125ºF.
7Boil potatoes in salted water for about 10 minutes until nearly tender, drain, and shake in colander to rough up edges.
8Transfer beef and vegetables to a platter and tent with foil; increase oven temperature to 450ºF.
9Toss potatoes with melted fat, 1 teaspoon salt, and garlic powder on a separate rimmed baking sheet and roast for 60 to 80 minutes, stirring every 20 minutes.
10Prepare horseradish sauce by whisking lemon juice, chopped chives, crème fraîche, prepared horseradish, water, and salt together.
11Whisk 1/4 cup beef broth with cornstarch; combine remaining broth, pan drippings, and Worcestershire sauce in a saucepan, bring to a boil, whisk in cornstarch slurry, and simmer for 8 minutes until thickened.
12Slice beef across the grain and serve with vegetables, potatoes, horseradish sauce, and gravy.
Ingredients
1 whole beef rump or round roast
2 1/2 tsp kosher salt
1/4 tsp freshly ground black pepper
1 lb carrot, peeled
1 lb parsnip, peeled
3 clove garlic, finely grated
3 sprig fresh rosemary, leaves picked and finely chopped
3 tbsp olive oil
3 lb Yukon Gold potato, peeled and cut into 1 1/2-inch chunks