English Sunday Roast

Name: English Sunday Roast
Ingredients: 1 (about 3-pound) beef rump or round roast, preferably tied with twine; 2 1/2 teaspoons kosher salt, plus more as needed; 1/4 teaspoon freshly ground black pepper, plus more as needed; 1 pound medium carrots (6 to 8); 1 pound medium parsnips (4 to 5); 3 cloves garlic; 3 sprigs fresh rosemary, plus more for garnish if desired; 3 tablespoons olive oil, divided; 3 pounds Yukon Gold potatoes; 1 tablespoon plus 1 teaspoon kosher salt, divided plus more as desired; 1/3 cup melted duck fat, chicken fat, butter, beef tallow, bacon fat, or olive oil; 1/2 teaspoon garlic powder; 1/2 medium lemon; 1 small bunch fresh chives; 1/2 cup crème fraîche or sour cream; 1/4 cup prepared horseradish; 1 tablespoon water; 1/2 teaspoon kosher salt, plus more as needed; Freshly ground black pepper; 2 cups beef broth, divided; 2 tablespoons cornstarch; 2 tablespoons Worcestershire sauce; Yorkshire pudding; Flaky salt, for sprinkling