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 a