Preheat the over to 200 degrees celsius. Place the mushrooms on a baking tray, stalk side up and season with freshly ground black pepper. Lightly spray with olive oil, cover with foil and roast for 20 minutes or until soft.
Meanwhile, to make the mustard crust, place all the ingredients in the food processor and whiz to a smooth paste. Rub the crust into both side of the steaks and set aside for 5 minutes.
Heat a chargrill or barbeque grill to medium. Cook the steaks for 3-4 minutes each side or until cooked to your liking. Transfer to a plate, cover lightly with foil and leave to rest for 5 minutes before serving with mushrooms and a rocket salad or other green vegetables.