Make the beef marinade by mixing all the ingredients together. Rub the mixture over the beef fillet, and leave to marinate overnight, of for 5 hours. When ready, heat a griddle pan and quickly brown the beef fillet all over to seal the meat. Place the sealed meat in an oven proof dish then continue to cook in the oven for about 10minutes (or to your liking).Take the cooked fillet out of the oven, and leave to rest while you put together the salad. Place the salad leaves, coriander leaves, cashew nuts, spring onions and mandarin segments together in a large bowl. Make the dressing with the juice of the lime, sesame oil, rice vinegar, fish sauce and fresh seasoning. Mix the dressing with the salad and divide onto 2 serving plates. Slice up the cooked beef fillet carefully, and arrange in the shape of a fan on one side of the plates. Sprinkle with the black sesame seeds just before serving.