To make the dressing, place the coconut milk, fish sauce, lime juice and chilli in a bowl and stir to combine.
Heat a char-grill pan over high heat. Brush the steak with oil and cook for 4 minutes each side for medium-rare or until cooked to your liking. Allow to rest and slice.
Combine the steak, basil, coriander, mint, tomato, onion and cucumber in a large bowl. Divide the salad between serving bowls and serve with the dressing on the side. Serves 4.