Sprinkle both sides of the beef with salt and pepper. Grill or barbecue each side for 3–4 minutes (depending on the thickness of the steak), turning occasionally. Fat should drip off the meat and the meat should cook slowly enough to remain juicy and not burn. You can pan-fry the meat if you prefer. Rest it for 4–5 minutes before thinly slicing.
Mix the beef slices with the fish sauce, lime juice, shallots, spring onions, coriander and chilli powder. Taste and adjust the seasoning.
Spoon over a pile of mixed salad leaves and garnish with a few mint or coriander leaves and chilli slices.