Pat the steaks dry. Heat 1 tablespoon of oil in a wok or frying pan. Fry the steaks for 3-4 minutes on each side. Season with salt and pepper. Wrap in foil and leave to rest for about 10 minutes.
Meanwhile wash the cucumber and cut it into pieces of about 5 cm. Cut cucumber pieces first lengthwise into slices, then into strips. Peel onions and slice or cut into thin rings. Clean and wash spring onion and cut into thin rings.
For the vinaigrette of lemon grass, remove the outer leaves, cut the rest very finely. Wash the chilli and cut into thin rings with the seeds. Wash mint, shake dry, pluck off leaves and cut coarsely.
Squeeze the limes. Mix lime juice, fish sauce, lemon grass, palm sugar, chilli and mint. Fold in 2 tablespoons of oil.
Cut the steaks into thin slices. Mix the prepared salad ingredients with the chilli and mint vinaigrette and leave to stand for about 30 minutes.