Season the 2 beef rump steaks, add to a very hot pan and cook for 1-4 minutes each side, depending on how you like your steak done. Transfer to a plate, loosely cover with foil, and set aside to rest for 5 minutes.
While the steak is resting, slice the large mushrooms and add them to the same hot pan. Stir-fry for 2-3 minutes, until beginning to wilt. Reduce the heat to low.
Add the soya sauce to a small saucepan and bring to a nearly boiling point
Remove from the heat, mix in the greek yogurt, add white pepper to taste.
Divide the steaks between plates and spoon over the mushroom and the pepper sauce.
*Serve with cooked potatoes, green beans or peas, salad and oven chips. (optional)