Ingredients

Step by step

1. Heat the oven to 220 ̊C, fan 200 ̊C, gas 7. Take the beef fillet out of the fridge to come to room temperature. Parboil the potato cubes for 5 minutes in a large pan of boiling salted water, then drain and leave to steam dry for a minute or two.
2. Tip the potatoes onto 1 very large or 2 smaller roasting trays, so that they are spread out in a single layer. Pour over 60ml of the sunflower oil, season generously and turn to coat. Roast on the top shelf of the oven for an initial 30 minutes until starting to turn crisp and golden, giving them a stir after 15 minutes.
3. For the mayonnaise, put the egg yolks, lemon zest, mustard and white wine vinegar into the bowl of a small food processor or a liquidiser. With the motor running, very very slowly pour in 240ml sunflower oil from a measuring jug; as you add the oil, it will begin to emulsify and you will be left with a thick mayonnaise. Scrape down the sides of the food processor every now and then. Once all the oil has been incorporated, with the motor still running, add the truffle oil to taste, plus a little lemon juice if you wish. Season then set aside in the fridge, covered.
4. Toast the peppercorns in a large dry nonstick frying pan until smelling fragrant, then grind coarsely in a pestle and mortar. Tip the ground peppercorns into a roasting tin that is big enough to hold the beef.
5. Season the beef fillet all over with salt and rub with a splash of oil. Add to the hot frying pan and brown on all sides, around 6 minutes in total. Transfer to the roasting tin and press into the peppercorns to coat all over the fillet.
6. Once the potatoes have had 30 minutes, reduce the oven temperature to 190 ̊C, fan 170 ̊C, gas 5. If you only have room for 2 shelves in your oven and have the potatoes spread between 2 trays, you can combine them into a single tray now.
7. Roast the beef below the potatoes for 15-20 minutes, depending on whether you like it to be rare or medium rare. If you have a temperature probe, the centre should be 50-55°C. Cover with foil and leave to rest for 15 minutes before slicing. At this point, if the potatoes are crispy and golden brown, turn off the oven and keep them warm inside, otherwise let them cook for a little longer while the beef rests.
8. Toss the watercress in a squeeze of lemon juice and season to taste. Once the fillet has rested, slice into 6-8 even rounds – 1 per person. Divide between plates, spoon over the steak juices and serve with the crispy potatoes, watercress and a good spoonful of the truffle mayonnaise on the side.