Preheat the oven to 350°F (180°C).
Blanch the sorrel in boiling salted water, refresh it immediately, then strain well. Process the sorrel and olive oil in a food processor. Season with salt and pepper.
Heat 2 tablespoons of the extra olive oil in a frying pan. Add the shallot and cook over medium heat until softened. Add the garlic, tomato, olives and pine nuts. Combine well and allow the mixture to stew for about 10 minutes. Season with salt and pepper.
Heat the remaining olive oil in a flameproof roasting tin. Add the beef and brown on all sides over high heat. Transfer to the oven and roast the beef for 10 minutes, then remove and add the tomato mixture and scatter over feta. Return to the oven and roast for a further 10 minutes.
Remove the roast from the oven and allow the beef to rest in its juices, covered with foil, for 10 minutes.
Drizzle over the sorrel oil before serving.