Roast Beef with Shallots

Name: Roast Beef with Shallots
Ingredients: 1 bottom round roast (5 pounds), trimmed and tied; Kosher salt and freshly ground pepper; 6 tablespoons extra-virgin olive oil; 1 pound shallots, peeled and trimmed, halved if large; ¼ cup chopped garlic (from 1 head); ½ cup Dijon mustard; 2 handfuls sage, oregano, rosemary, or thyme sprigs, or a combination; ½ cup dry vermouth, such as Dolin; ⅔ cup heavy cream; 2 cups sour cream; ⅛ teaspoon cayenne pepper; 2 tablespoons drained prepared horseradish; Mostarda, for serving (optional)