Step 1

Season beef with 2 tablespoons salt and 1 teaspoon pepper. Wrap with plastic and refrigerate at least 8 hours and up to 1 day. Unwrap and let stand at room temperature 1 hour.

Step 2

Preheat oven to 350 degrees. Heat a large skillet (preferably cast iron) over medium-high. Add 2 tablespoons oil and brown beef all over, 12 to 14 minutes. Transfer to a plate; carefully pour oil from skillet and discard.

Step 3

Place shallots in a single layer in bottom of skillet; drizzle with 1 tablespoon oil. Stir together garlic, mustard, and 2 tablespoons oil; rub mixture all over beef. Lay 1 handful of herbs down center of skillet. Place beef on top, fat-side up. Transfer to oven and roast 45 minutes.

Step 4

Place remaining handful of herbs on top of beef; drizzle herbs with remaining 1 tablespoon oil. Return to oven and roast until a thermometer inserted in thickest part of roast reaches 105 degrees for rare, 10 to 20 minutes. Transfer roast to a cutting board and shallots to a serving platter, loosely tent both with foil, and let stand at least 30 minutes.

Step 5

Return skillet to medium-high heat. Stir in vermouth; bring to a boil and cook, stirring occasionally, until just a few tablespoons remain, 5 to 7 minutes. Strain through a fine-mesh sieve and let cool 10 minutes.

Step 6

Whip heavy cream to stiff peaks. Fold in sour cream, cayenne pepper, horseradish, and vermouth mixture. Season with salt. Store horseradish cream in refrigerator until ready to serve, or up to 3 days.

Step 7

Remove twine from roast; thinly slice across the grain and serve, with horseradish cream, shallots, and mostarda.