While the tenderloin is in the refrigerator, preheat the oven to 375°F. Cut off the top of the head of garlic, exposing all of the tips of the cloves. Drizzle with 1 tsp olive oil and wrap tightly in a piece of aluminum foil. Place in the oven until the garlic is caramelized, soft, and golden brown, 20-30 minutes. Once caramelized, remove from the oven and allow to cool.
1 head garlic
In a 4-quart saucepan, combine the onion, beef stock, red wine, thyme, rosemary, and salt. Bring to a boil and reduce to a low simmer. Allow the mixture to simmer until the liquid is reduced by half, 45-55 minutes. Once reduced, combine the water and cornstarch. Whisk until smooth and then whisk into the reduced sauce. Return the sauce to a simmer to activate the cornstarch and thicken slightly. Keep warm until ready to serve.
¾ cup minced red onion, 1 ½ cups beef stock, 1 ½ cups red wine, 1 sprig thyme, 1 sprig rosemary, 1 ½ tsp salt, 1 tbsp water, 2 ½ tsp cornstarch
After 1 hour, remove the beef tenderloin from the refrigerator. Reduce the oven temperature to 325°F. Heat 2 tbsp of neutral oil (grapeseed oil works well) in a 12-inch skillet over medium heat. When the oil is hot and shimmering, add the beef tenderloin. Sear the tenderloin on all sides to form a crust. The tenderloin will stick to the skillet at first, but once it easily releases, it is ready to turn, 2-3 minutes per side. Once seared, remove from the heat.
1 tbsp grapeseed oil (vegetable also works)
In a small bowl, squeeze out the roasted garlic from the husk. Mix with the coarse black pepper to form a paste. Rub the paste all over the seared tenderloin. Place the tenderloin in the oven and roast until the desired internal temperature is reached (125°F for medium-rare, 130°F for medium, 140°F for well), 30-40 minutes. Remove from the oven and allow the meat to rest for 15 minutes before slicing. Serve with the warm sauce.