Place the peppercorns in a pestle and mortar and lightly crush down, then tip on to a flat plate. Pat the steaks dry with kitchen roll and press the mixed peppercorns on to both sides of the steaks, using your hands. If you have time, cover with foil or clingfilm and leave at room temperature for 2-3 hours to allow the flavours to infuse.
Heat the oil in a large heavy-based frying pan over a high heat. Add the steaks and cook for 2 minutes on each side, then reduce the heat, add half the butter and cook the steaks for 5-10 minutes, turning once, depending on how rare you like it. Transfer the steaks to a plate, season with salt and set aside in a warm place to rest while you make the sauce.
Pour away any excess fat from the pan, add the remaining butter and fry the shallots for a few minutes until softened but not browned. Add the stock, Worcestershire sauce and cook rapidly, scraping the bottom of the pan with a wooden spoon to release any sediment. Stir in the mustard, cream and green peppercorns, then season to taste with salt and just warm through. Heat the brandy in a ladle over a flame and wait until it ignites, then pour over the cream, stirring to combine.
Arrange the steaks on warmed plates, stirring any juices from the rested steaks into the peppercorn sauce. Drizzle the peppercorn sauce over the steaks and serve at once with the chips and watercress salad.