Roast Beef with Horseradish Sauce

Preheat oven to 375 degrees. Sprinkle beef with 1/2 teaspoon salt, and season with pepper. Heat oil in a large ovenproof saute pan over high heat until hot but not smoking. Add beef; brown on all sides, about 5 minutes total. Remove pan from heat.

Place onion halves in pan, cut sides down. Transfer pan to oven. Cook beef and onions until an instant-read thermometer inserted into center of beef registers 140 degrees (for medium-rare), 35 to 40 minutes. Transfer beef to a wire rack set over a rimmed baking sheet, and let stand 10 minutes before carving. Reserve onions.

Stir together sour cream, horseradish, lemon juice, 1/4 teaspoon salt, and pepper to taste in a small bowl. Serve beef with horseradish sauce and roasted onions.