Mix the vegetable oil with the five-spice powder, then brush this mixture all over the steaks. Set the steaks aside on a plate, whilst you prepare the dressing and salad vegetables.
Put all the ingredients for the dressing into a large bowl and mix until combined. Taste and adjust the flavour to your liking.
Add all the salad vegetables to the dressing in the bowl and tumble everything together to mix and coat. Pile the salad onto two serving plates and set aside.
Heat a heavy-based, non-stick frying pan or griddle pan over a high heat until very hot. Add the steaks to the pan and sear on both sides for a couple of minutes or so until cooked to rare, or cook for slightly longer, if you prefer.
Remove the steaks to a warm plate, cover with foil and leave to rest briefly, a couple of minutes, before thinly slicing into strips. Arrange the slices of beef on top of the salad.
To serve, scatter the beef salad with the chopped herbs, chilli slices and sesame seeds and serve immediately.