Combine dressing ingredients in a bowl and whisk well together. Add in steak to marinate. Cover with cling film wrap and refrigerate, turning occasionally for 1-2 hours to develop the flavour.
Remove steak from the marinade and grill for 2-3 minutes each side (for medium doneness) or until cooked to your liking. Remove to a plate and cover with foil and leave aside to set for 10 minutes. Thinly slice steak across the grain.
Put cucumber, tomatoes, onion, chilli, coriander, mint, basil, kaffir lime leaves and peanuts in a large mixing bowl. Add in steak slices and drizzle in remaining dressing ingredients and gently toss well to combine. Serve immediately.