Put the steak on a board and cut off all the hard fat. Cut the steak in half to give 2 portions - each should weigh about 140g by the time you've trimmed off the fat. Put the peppercorns in a pestle and mortar and pound until coarsely ground. Stir in the salt.
Pour a teaspoon of the oil into a small dish or eggcup, dip the end of a pastry brush in the oil and brush the steaks on both sides. Coat the steaks generously with the pepper and salt mixture.
Cut the tomatoes in half and place them on a grill pan lined with foil, cut side up. Season the tomatoes with a little ground black pepper, then cook under a preheated hot grill for 5-8 minutes until softened.
Pour the remaining oil into a small non-stick frying pan. Place over a high heat and stir-fry the mushrooms for 2-3 minutes until lightly browned. Make sure you keep the heat up high so they get some lovely colour. Tip the mushrooms into a bowl and return the pan to a medium-high heat.
Put the steaks in the pan and cook for 2-3 minutes on each side or according to taste. Two minutes on each side should give you a medium-rare steak.
Put the steaks on warmed plates and leave them to rest. Return the mushrooms to the pan and warm through over a high heat. Dissolve the stock in the just-boiled water and add to the pan, stirring constantly to lift the sediment and juices from the bottom of the pan.
Add the creme fraiche, mustard, and wine, if using, and stir together for 1-2 minutes until the sauce is hot and has thickened slightly. Spoon the mushrooms and sauce over the steaks and serve with the grilled tomatoes and a large, lightly dressed salad.