Parboil the potatoes for 5 minutes, then drain. Cut in half lengthways, then cut into chunky wedges. Put 1 tablespoon of the oil in a bowl, add the egg white, mustard, turmeric and a little pepper and fork together until the egg white is slightly frothy. Add the potatoes and toss well.
Arrange the potato wedges in a single layer on the flat plate of a hot gas barbecue or in a ridged pan on a charcoal barbecue and cook for 10 minutes. Add the tomatoes and cook for 5 more minutes or until the chips are golden and the tomato skins just beginning to split.
Meanwhile, brush the mushrooms and steaks with the remaining oil and sprinkle the steaks with a little pepper. Cook the steaks on the barbecue for 3—4 minutes on each side, or until cooked to your liking. Set aside to rest for 5 minutes while cooking the mushrooms.
Transfer the steak, chips and vegetables to serving plates and serve immediately.