Step 1

Pat 1 (about 3-pound) beef rump or round roast dry with paper towels. Season all over with 2 teaspoons of the kosher salt and 1/4 teaspoon black pepper. Let sit at room temperature for at least 1 or up to 2 hours. Meanwhile, peel 1 pound medium carrots and 1 pound medium parsnips. Halve crosswise, then halve or quarter each piece lengthwise so they are all about the same size.

Step 2

Arrange a rack in the middle of the oven and heat the oven to 375ºF.

Step 3

Finely grate 3 garlic cloves. Pick the leaves from 3 fresh rosemary sprigs, then finely chop (2 to 3 tablespoons). Place the garlic, rosemary, and 1 tablespoon of the olive oil in a small bowl and stir to combine. Rub the mixture all over the beef. Place in the middle of a roasting pan or rimmed baking sheet fat-side up.

Step 4

Scatter the carrots and parsnips around the beef. Drizzle with the remaining 2 tablespoons olive oil, season with the remaining 1/2 teaspoon kosher salt and a few grinds of black pepper, and stir to coat. Arrange into an even layer.

Step 5

Roast until the outside of the roast is browned, about 30 minutes. Reduce the oven temperature to 275ºF. Roast until the beef is medium rare (registers 125ºF on an instant-read thermometer in the thickest part), about 1 hour more, checking the temperature after 30 minutes. Meanwhile, boil the potatoes.

Step 6

Peel and cut 3 pounds Yukon gold potatoes into 1 1/2-inch chunks. Place in a large saucepan or pot and add enough cold water to cover by about 1 inch. Add 1 tablespoon of the kosher salt and bring to a boil over medium-high heat. Reduce the heat to maintain a simmer and cook until the potatoes are nearly knife tender but not totally soft, about 10 minutes. Drain the potatoes in a colander, then shake the colander to rough up the edges of the potatoes.

Step 7

When the beef is ready, transfer the beef and vegetables to a serving platter with tongs. Tent the platter with aluminum foil to keep warm. Reserve the roasting pan or baking sheet and the pan drippings. Increase the oven temperature to 450ºF.

Step 8

Place 1/3 cup melted duck fat, chicken fat, butter, beef tallow, bacon fat, or olive oil; the remaining 1 teaspoon kosher salt; and 1/2 teaspoon garlic powder on a rimmed baking sheet (do not use the one from the roast). Add the potatoes and gently toss to coat. Arrange in a single layer. Roast, stirring every 20 minutes, until golden-brown and very crisp, 60 to 80 minutes total. Meanwhile, make the horseradish sauce and gravy.

Step 9

Juice 1/2 medium lemon into a medium bowl until you have 1 tablespoon. Finely chop 1 small bunch chives until you have 1 tablespoon. Add the chives, 1/2 cup crème fraîche or sour cream, 1/4 cup prepared horseradish, 1 tablespoon water, and 1/2 teaspoon kosher salt to the bowl, and stir to combine. Taste and season with more kosher salt and freshly ground black pepper as needed.

Step 10

About 20 minutes before the potatoes are ready, place 1/4 cup of the beef broth and 2 tablespoons cornstarch in a small bowl and whisk until smooth. Pour any drippings from the roasting pan or baking sheet into a small saucepan. Add the remaining 1 3/4 cups beef broth and 2 tablespoons Worcestershire sauce. Bring to a boil over medium-high heat.

Step 11

While whisking constantly, add the cornstarch mixture. Reduce the heat to medium and simmer until the gravy thickens enough to coat the back of a spoon, about 8 minutes. Taste and season with kosher salt and black pepper as needed. Remove the saucepan from the heat and cover to keep warm.

Step 12

If desired, return the beef, carrots, and parsnips to a 200ºF oven to rewarm. Transfer the beef to a clean cutting board and cut across the grain into 1/4-inch-thick slices. Serve with the carrots, parsnips, potatoes, gravy, horseradish sauce, and Yorkshire pudding. Lightly sprinkle the beef with flaky salt and garnish with fresh rosemary sprigs if desired.