01 -
Tie the beef tenderloin with kitchen twine to ensure even cooking. Place it on a wire rack set on a baking sheet. Season the tenderloin liberally with kosher salt and freshly cracked black pepper, then refrigerate uncovered overnight.
02 -
Remove the tenderloin from the refrigerator at least 1 hour before roasting. Preheat the oven to 225°F (about 107°C). Tuck fresh sage, rosemary, and thyme underneath the kitchen twine all over the tenderloin.
03 -
Place the beef on a baking sheet and roast in the center of the oven for 2 1/2 to 3 hours, or until the internal temperature reaches 120°F to 125°F (about 49°C to 52°C). Remove the tenderloin and rest for 10 minutes.
04 -
Melt the unsalted butter in a saucepan over low heat until it just begins to brown slightly, then remove from heat. Adjust the oven rack to 6 inches (15 cm) from the broiler and set it to high. Spoon some melted butter over the beef, then broil for 2-3 minutes, turning the tenderloin every 30 seconds and basting with butter to ensure even browning. Remove from oven and drizzle with remaining butter. Let the beef rest for 10 minutes before slicing.
05 -
Heat a skillet over low heat and melt the butter. Add sliced sweet onions and salt, then stir. Cook, stirring occasionally, until onions turn golden brown and caramelized, about 45 to 60 minutes. Sprinkle in chopped thyme and stir after 45 minutes.
06 -
In a bowl, whisk together sour cream, heavy cream, prepared horseradish, mayonnaise, Dijon mustard, lemon juice, salt, and pepper. Refrigerate for 30 to 60 minutes. Top with fresh chives before serving.