1. Trim the silver skin off the beef tenderloin. Tie with kitchen twine every 1.5 inches for even cooking. Let it sit at room temperature for 30 minutes before cooking.

2. Melt butter in a large saucepan over medium heat. Add sliced onions, thyme, and a pinch of salt. Cook slowly, stirring occasionally, for 30–40 minutes until the onions are deeply golden and soft.

3. Preheat oven to 425°F. In a cast iron skillet, heat olive oil over medium-high heat. Sear the tenderloin on all sides until browned, about 3–4 minutes per side. Transfer to a rack in a roasting pan.

4. Insert a meat thermometer into the thickest part of the tenderloin. Roast for 20–25 minutes or until internal temperature reaches 130°F for medium-rare. Let rest for 15 minutes before slicing.

5. In a small bowl, mix sour cream, horseradish, mayonnaise, and Dijon mustard. Season with salt and pepper to taste. Chill until ready to serve.

6. Slice the tenderloin into medallions. Top with caramelized onions and a dollop of horseradish sauce. Garnish with fresh thyme.