1. Pat the beef tenderloin dry with paper towels. Season generously with kosher salt and black pepper on all sides.

2. Heat olive oil in a large ovenproof skillet over medium-high heat. Sear the beef on all sides until deeply browned, about 3–4 minutes per side.

3. Transfer the skillet to a preheated 425°F oven. Roast for 20–25 minutes or until internal temperature reaches 125°F for medium-rare. Remove from oven and let rest for 15 minutes before slicing.

4. Meanwhile, in a separate large skillet over medium-low heat, melt butter. Add sliced onions and a pinch of salt. Cook, stirring frequently, for 30–40 minutes until caramelized.

5. Add minced garlic and fresh thyme to the onions in the final 5 minutes of cooking. Stir and cook until fragrant.

6. In a small bowl, whisk together sour cream, horseradish, Dijon mustard, white wine vinegar, salt, and pepper. Adjust heat level to your taste. Chill until serving.

7. Slice rested tenderloin into thick medallions. Serve with caramelized onions and a dollop of horseradish sauce on the side. Garnish with thyme if desired.