For the steaks, spread the coarse black pepper pieces over a small plate. Coat all sides of the steaks with the Dijon mustard and then press them into the crushed peppercorns to coat.
Heat a large frying pan until very hot and add the sunflower oil. Season the steaks with salt, then add to the pan to cook for 1-2 minutes. Turn once to brown both sides (don't move the steaks around once they are in the pan or the peppercorn crust will fall off - cook to achieve a good crusty coating on each side.)
Add the butter and allow it melt and turn nut-brown in colour, but don't let it burn.
Add the mushrooms and stir in the butter. As the mushrooms start to absorb the pan juices, turn the steaks again and allow them to cook for 3-4 minutes on each side, or until cooked to your liking.
Remove the steaks and place onto a baking tray and leave to rest in a warm place.
For the tatties, place the potatoes into a large mixing bowl. Add the butter and gently crush each potato with a fork until it just splits. Season, to taste, with salt and freshly ground white pepper, and add the parsley. Mix until all the butter has been absorbed, but don't over-mash the potatoes.
For the spinach, heat a medium frying pan or wok until hot. Add the butter and the spinach and stir-fry for one minute, then add the water. Continue to stir until the spinach has wilted. Remove from the heat, season, to taste, with salt and freshly ground white pepper and set aside until ready to serve.
For the whisky sauce, add the whisky to the pan used to cook the steaks, and cook over a very high heat for one minute to boil off the alcohol. (CAUTION: the whisky may catch fire, which is perfectly acceptable, but if this worries you, have a large lid handy to place onto the pan to prevent the flames. And make sure you have your extractor fan turned off.)
Add the stock and boil to reduce the liquid volume, until thickened, then add the cream. Boil to reduce the liquid volume again, scraping and stirring to loosen the stuck bits from the bottom of the pan.
Pour any juices released by the resting steaks into the sauce and stir well.
To serve, place a chefs' ring into the centre of each of four plates. Fill with the crushed potatoes and carefully remove the ring. Place a small pile of spinach onto the potatoes, and arrange two medallions of steak on top of each pile. Spoon the sauce and the mushrooms over the steak, and serve.