1. Preheat oven to 180*C
2. Trim and truss beef fillet and lightly oil and season with pepper. Set aside
3. Roughly chop lemongrass taking care to only use the white young stems. Place lemongrass in a mortar and pestle, and pound for a minute or two. Add the peeled garlic and ginger and continue to pound until you have a rough paste.
4. Finely chop the roots or the coriander (reserve the leaves for later) and the spring onions, also slice your chillies (it is up to you if you leave the seeds in or out depending on how hot you like it).
5. Preheat a large frying pan and sear your beef taking care to get a nice even colour on all sides.
6. Put the beef in a roasting tray and roast in the preheated oven for 14 minutes (for rare). While the beef is in the oven, use the same pan which you seared your beef in for making the sauce.
7. Fry your lemongrass paste on a medium to high heat in a little peanut oil until softened and fragrant.
8. Add in spring onions, coriander roots and half the sliced chilli and continue to fry for a few minutes.
9. Measure in the soy sauce and fish sauce, then reduce a little before adding the tin of coconut cream. Allow the sauce to simmer gently.
10. Once the beef is done, allow to rest for half the time it was in the oven. Remove the truss string and place on a serving platter. Pour the sauce over and around the beef, garnish with the extra chilli, coriander leaves and squeeze the finger lime pearls so they fall in a line down the middle of the beef