Combine the cucumber, tomato, onion, capsicum, coriander and mustard in a medium bowl. Put aside.
Heat a heavy nonstick frying pan over high heat. Brush the steaks with oil on both sides and add to the pan. Cook for 2 to 3 minutes each side for rare; reduce the heat to moderate and continue cooking for 2 to 3 minutes each side for medium; cook for a further 4 to 6 minutes each side for well done.
Serve with the salsa alongside. Accompany with the oven fries, if liked.