Trim the fat from the steaks and rub the meat with the crushed peppercorns and salt flakes.
Mix together the yogurt, horseradish sauce and garlic and season to taste with salt and 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 until browned. Turn over and cook for 2 minutes for medium rare, 3–4 minutes for medium or 5 minutes for well done.
Spoon the salad leaves into the centre of six serving plates. Thinly slice the steaks and arrange the pieces on top, then garnish with the remaining red onion.