Brush the steak evenly with oil and season to taste. Heat a griddle pan until smoking, then add the steak and cook for 3-6mins each side, or until it has been cooked to your liking. Transfer to a plate and allow to rest for 5 minutes.
Meanwhile, place all the salad ingredients in a bowl. Whisk together all the dressing ingredients in a small bowl, then pour half of it over the salad and mix well.
Using a sharp knife, cut the cooked steak into thin slices and arrange on top of the salad. Pour over the remaining dressing, scatter over the peanuts and fresh mint leaves, and serve immediately.