Preheat your oven to 425˚F.
Place a large skillet over high heat. Brush two tablespoons of olive oil all over the beef and season with salt and pepper. Sear the beef in the hot skillet for about a minute on each side, or until the beef has a lovely dark crust all over it. Transfer to your roasting tin.
Combine the finely chopped rosemary, garlic and remaining 4 tablespoons of olive oil. Spread all over the beef. Roast your beef in the preheated oven until a meat thermometer inserted into the thickest part of the tenderloin reads 125 – 135˚F for rare to medium rare. This should take approximately 30 – 40 minutes.
While the beef is cooking, start making your sauce. Melt the butter in a medium-sized pot over medium high heat. Add the sliced shallots and fry until they start to turn golden, about 8 minutes, being sure to stir frequently so they do not burn. Add the black pepper and red wine and bring to a boil. Reduce by two-thirds. Add the beef stock and boil gently until the liquid has reduced by half. Taste the sauce and add any salt or pepper you would like. If you find your sauce is not thickened to your liking, you can add a slurry of cornstarch and water at this point (see cook’s note).
I love to roast potatoes in the same tin as this beef. Simply peel and quarter your roasting potatoes. Par-boil in boiling water until they are almost cooked (a knife can be easily inserted but holds the potato). Drain and add to the roasting tin along with the beef. Feel free to also toss the potatoes in some of the rosemary and garlic mixture.
Stocks can vary so it is important to look at your sauce and decide whether you need to thicken it with a mixture of cornstarch and water. If you find that the sauce is too thin, whisk together 1 teaspoon of cornstarch and 2 tablespoons of cold water in a small bowl until smooth. Stir the mixture into the sauce and allow the sauce to simmer over medium heat, stirring continuously, for at least 5 minutes and until thickened