For the beef, brush the steaks with oil and season with salt and freshly ground black pepper. Heat a ridged griddle pan until smoking, then add the steaks and griddle for 3-6 minutes on each side, or until cooked to your liking. Remove from the heat and leave to rest in a warm place for five minutes.
For the salad, finely slice the Chinese leaf and place into a large bowl. Cut the tomatoes into wedges and add to the bowl. Finely slice the shallots and add to the bowl.
Slice the cucumber in half down the middle. Remove the seeds with a teaspoon and slice the cucumber flesh on the diagonal. Discard the seeds and add the cucumber to the bowl. Slice the spring onions on the diagonal and add to the bowl.
Tear the basil leaves and add to the bowl. Tear the coriander leaves and stalks and add to the bowl. Mix together to combine well.
For the dressing, grate about two teaspoons of the palm sugar into a small clean bowl. Roll the limes to release the juices. Cut in half and squeeze the juice into the bowl with the sugar. Stir until the sugar is dissolved.
Add the fish sauce to the dressing. Taste the dressing and adjust the flavours, adding more lime juice, fish sauce or palm sugar to taste. Finely chop the chillies and add to the dressing.
Cut the cooked steak into thin slices on the diagonal. Add the sliced steak to the salad ingredients, drizzle over the dressing and mix together well. Add to the bowl the juices from the plate the meat was resting on.
Serve the salad on a large plate, with lime wedges on the side.