Roasted Beef Tenderloin with Caramelized Onions and Horseradish Sauce - Recipe By Me

Ingredients

• Olive oil, as needed
• 3 tablespoons unsalted butter
• 2 1/2 pounds yellow onions, thinly sliced
• Kosher salt, to taste
• Freshly cracked black pepper, to taste
• 2 large fresh thyme sprigs
• 2 large fresh rosemary sprigs
• 1 bay leaf
• 1/4 cup sherry wine
• 6 ounces mayonnaise
• 6 ounces sour cream
• 3 ounces prepared horseradish
• 1/2 lemon, zested and juiced
• 3 pounds whole beef tenderloin, trimmed
• 1 tablespoon kosher salt
• 1 tablespoon granulated garlic
• 2 teaspoons freshly cracked black pepper
• Fresh thyme leaves, for garnish