Season the meat well with the pepper and seal in a non-stick pan over a high heat until well browned on both sides but rare inside. (Depending on the pan used, you may need to brush the pan surface lightly with oil to prevent the meat from sticking). Transfer the meat to a plate and allow it to rest for 4 to 5 minutes to let the meat relax and help keep the juices in the meat.
Slice the meat thinly across the grain and place in a bowl.
Add the mixed lettuce leaves, cucumber, cherry tomatoes and mint.
To make the dressing, place the garlic, coriander, chillies, lime juice, fish sauce and spring onions in a food processor and process until well blended, about 1 minute.
Toss the beef and salad vegetables with the dressing and serve at once.
Notes: Slice the meat across the grain to achieve the most tender eating quality. This is important when cutting raw meat and when carving roasts.