Rub the steaks with the crushed peppercorns and salt flakes.
Mix together the yoghurt, horseradish sauce and garlic and season to taste with salt and freshly ground black pepper. Add the salad leaves, mushrooms and most of the red onion and toss gently.
Heat the oil in a frying pan, add the steaks and cook over a high heat for 2 minutes, or until browned. Turn over and cook for a further 2 minutes for medium rare, 3-4 minutes for medium or 5 minutes for well done. Place the steak on a warm plate and allow to rest for a few minutes.
Spoon the salad leaves into the centre of 2 serving plates. Thinly slice the steaks and arrange the pieces on top. Garnish with the remaining red onion.