Preheat the oven to 210°C. Season the meat with the salt and pepper.
In a large frying pan, heat two teaspoons of the oil over medium-high heat until good and hot, then add the meat and sear until well browned on all sides, about ten minutes total.
Transfer the meat to a rack set on a baking tray. Roast until an instant-read thermometer inserted in the thickest part registers 60°C for medium-rare, about 30 minutes, or to your desired degree of doneness. Remove it from the oven, cover with aluminum foil, and let rest until the sauce is nearly done before slicing.
While the meat cooks, make the sauce. Heat the remaining 2 tsps of oil in a large saucepan over medium-high heat. Add the shallots, carrot and celery and cook, stirring a few times, until softened, about 5 minutes.
Add the garlic and cook for two minutes more. Add the wine and and stir in the tomato paste. Add the bay leaf and thyme and bring to a boil. Simmer until the liquid is reduced to about 120ml, about 40 minutes.
Strain through a fine mesh strainer into a small saucepan. Stir in the cocoa and rosemary and season with salt and pepper.
Serve on the side with the sliced tenderloin.
Per Serving: Calories 280; Total Fat 9g (Sat Fat 3g, Mono Fat 4g, Poly Fat 0g); Protein 31g; Carb 7g; Fiber 1g; Cholesterol 82mg; Sodium 375mg