Put the stock on to boil with the onion, fennel, carrot and herbs. Cover and simmer for 10 mins. Trim the beef fillet of any sinew. Do not tie it.
Season the beef all over, then lower into the simmering stock. Allow the stock to return to a gentle boil, then lower the heat until it is barely simmering. Cover, then cook for 18 mins for medium-rare, 22 mins for well done. Remove the pan from the heat, leave the meat to stand for 15 mins in the stock, remove and pat dry. Slice into 8 pieces. The stock can be strained, cooled and re-used.
Make the salad by scattering the watercress leaves over a small serving plate. Whisk together the oil, vinegar, mustard and some seasoning. Drizzle half over the leaves and scatter with the nuts. Arrange the meat on top and mix 2 tbsp of the stock with the remaining dressing. Pour over the beef, sprinkle with sea salt flakes and serve with steamed Jersey Royal potatoes.