Make sure the steak is dry before you begin. Using your hands, press the black and white peppercorns into both sides of the steak. If you have time, cover with foil/clingfilm and leave for the pepper flavour to infuse into the steak for 2-3 hours.
Pour the olive oil into a heavy-based frying pan and, over a high heat, seal the steak for two minutes on each side.
Reduce the heat, add half the butter to the pan and cook the steak for a further 5-10 minutes, turning one more time. You can reduce the cooking time if you like your steak quite rare. Once cooked to your liking, remove the steak from the pan and set aside to rest.
Pour away most of the fat remaining in the pan. Now add the remaining butter and the shallots. Over a medium heat, cook the shallots until soft, but not brown.
Pour in the Worcestershire sauce, brandy and stock. Cook rapidly, scraping the bottom of the pan to incorporate all the flavours.
Add the green peppercorns, mustard and cream. Season to taste.
To make the chips, peel the potatoes and cut them into chunky chips. Place in a bowl of water for a couple of minutes - this helps remove the starch. Pour the sunflower or vegetable oil into a deep-sided pan, making sure it is no more than a quarter full if you're using an open pan; no more than half full if using a deep-fat fryer. Heat to 160C/325F/Gas 2.
Drain the chips and dry them on kitchen paper before placing them in a wire basket and lowering them into the heated oil. Cook them in small batches so that they don't stick together. Cook for four minutes until cooked through but not coloured. Remove from the oil and drain.
Increase the temperature of the oil to 190C/375F/Gas 5. Tip the blanched chips back into the wire basket and then carefully lower into the heated oil. Cook for a minute or two until crisp and golden brown. Drain well on kitchen paper.
Slice the steak on the diagonal and add to the sauce. Stir to combine the meat juices with the pepper sauce, and to warm the meat through. Do not re-boil.
Serve with the chips.