Preheat your oven to 425°F (220°C).

Prepare the garlic: Slice the top off the head of garlic to expose the cloves. Drizzle with 1 tablespoon of olive oil and wrap tightly in aluminum foil. Roast in the oven for about 30-40 minutes, or until the cloves are soft and caramelized. Allow to cool before squeezing the cloves out of their skins.

While the garlic is roasting, prepare the tenderloin. In a small bowl, combine the roasted garlic, chopped oregano, salt, and black pepper, mixing well to create a paste.

Rub this garlic and oregano mixture all over the tenderloin, ensuring it is evenly coated.

In a large oven-safe skillet, heat the remaining tablespoon of olive oil over medium-high heat. Sear the tenderloin on all sides until browned, about 2-3 minutes per side.

Transfer the skillet to the preheated oven and roast the tenderloin for about 20-25 minutes for medium-rare, or until a meat thermometer registers 135°F (57°C). Remove from the oven and let the meat rest for 10-15 minutes.

While the tenderloin is resting, prepare the gravy. In the same skillet, add the sliced leeks and sauté over medium heat until softened, about 5 minutes.

Pour in the red wine, scraping up any browned bits from the bottom of the pan. Allow the wine to reduce by half, about 5 minutes.

Add in the beef broth and bring to a simmer. In a small bowl, mix the butter and flour to form a paste (beurre manié). Whisk this into the gravy to thicken, cooking for an additional 5 minutes until the gravy is rich and velvety.

Slice the rested tenderloin and serve with the caramelized leek gravy drizzled over the top.