Heat 2–3 tablepoons olive oil in a saucepan and cook the onion gently for 10 minutes. Remove from the heat and allow to cool. Mix with all the other ingredients except the beef. Set this mixture aside.
Heat the oven to 200°C. Coat the fillet with olive oil and season well with freshly ground pepper. Oil a frying-pan with a tablespoon of oil and heat it until smoking; put in the fillet and brown it for 1 minute on each side. Transfer to a roasting pan and roast for 10 minutes, turn down the oven to 180°C for a further 10 minutes, then remove the fillet, transfer it to a deep plate and cover it lightly with foil. Let it rest for 10 minutes, then slice the meat into thick slices – a lot of juice should have run out by now. Mix the chopped mixture into the juices and muddle the slices of beef in this gravy. Serve with chunks of bread or in baguette sandwiches.