For the beef, place the beef fillet onto a sheet of cling film and roll into a sausage shape. Place into the fridge overnight to set the shape.
Grind the peppercorns, salt and thyme together in a pestle and mortar. Remove the cling film from the fillet, then rub lightly with a tablespoon of the olive oil. Rub the pepper mixture into the beef. Heat a frying pan until very hot, then add the beef and sear on all sides.
Remove the beef from the heat. Using a long, sharp knife, slice the beef as thinly as possible. Place the slices onto a board and press with the flat side of the knife to extend each slice.
Take a large, cold serving plate and drizzle with the remaining olive oil. Rub the oil over the plate with your fingers and then spread the beef slices over the plate.
For the dressing, place the egg yolks, vinegar, water and parmesan into a bowl and beat well until pale. Gradually add the olive and vegetable oils, whisking all the time so that it doesn't split. When all the oil has been incorporated, stir in the cream and season, to taste, with salt and freshly ground black pepper.
To serve, squeeze the lemon juice over the meat, crumble over the gorgonzola and top with the watercress. Drizzle over 1-2 tablespoons of the dressing. Any leftover dressing can be kept, covered, in the fridge for up to two days and used as a dip for warm bread.