Whisk together the garlic clove, brown sugar, lime juice and the chilli. Add the Thai fish sauce. Toss the herb salad with the cucumber, radishes and the torn coriander leaves. Divide between 2 plates.
Pan fry the steaks, trimmed of fat, for 4-5 minutes each side, rest, slice thinly and put on top of the salad. Tip the steak juices into the dressing, pour over the salad and scatter with the roasted peanuts.