To make the nahm jihm, trim the roots from the coriander and rinse well. Pick the leaves and put to the side. Add the coriander roots, ginger, garlic, half the chillies, lime zest and juice, fish sauce and brown sugar to a food processor and process until well combined and finely chopped.
For the salad, place onion, cucumber, grapes, coriander leaves, mint, remaining chillies and ½ of the nahm jihm dressing in a large bowl and toss to combine, season with salt and pepper.
To cook the steaks, pre-heat the BBQ and then turn to medium heat. Lightly coat the steaks in olive oil, salt and pepper. Drain off any excess oil, then place the steak on the BBQ grill and sear for 2 minutes.
Turn the steak with the raw side still facing up to achieve crisscross char-grill lines and wait for tiny drops of blood to form on the surface, (this is for medium rare) then turn over.
Cook the steak on the second side for half the time as the first side, then remove and allow the steaks to rest for 5 minutes.
To serve, place salad on a serving platter. Slice steaks into thick pieces and arrange on top of the salad. Drizzle with the remaining nahm jihm dressing and then scatter with peanuts.