Brush the steaks with the oil and season with pepper. Heat a ridged cast-iron grill pan or non-stick frying pan over a medium-high heat until hot. Add the steaks and cook for 3 minutes on each side for medium-rare or 4 minutes on each side for medium. (These cooking times are for 2-cm steaks; adjust slightly for more or less than this thickness.) Remove the steaks from the pan and leave to cool for at least 15 minutes.
Meanwhile, make the dressing. Put the mustard, mayonnaise and yoghurt in a small bowl and stir until combined.
Combine the radishes, carrots, yellow capsicum, celery, spring onions, walnuts and raisins in a large bowl. Quarter and core the apples, then cut them into 2-cm cubes and toss them in the lemon juice. Add the apples to the bowl with half of the dressing and turn to coat everything well. Season with salt and pepper to taste.
To serve, pile the rocket on four plates and spoon the apple and vegetable salad alongside. Cut the steak into thin slices and arrange on top. Spoon the remaining dressing over the top or hand it round separately in a jug.