1. Cook the potatoes in boiling water for 8-10 minutes until just tender, and drain well.
2. Meanwhile, cook the steak on a non-stick griddle pan or over medium-hot barbecue coals for 3-4 minutes on each side until well browned but still slightly pink in the centre. Transfer to a plate and leave to rest for 5 minutes.
3. To make the chimichurri sauce, in a small bowl, mix together the chilli, garlic, parsley, oregano, olive oil, red wine vinegar and 3 tbsp cold water.
4. Slice or dice the cooked steak and place in a large bowl. Add the cooked potatoes, chimichurri sauce, cucumber, red onion and watercress. Toss together and serve warm.
Recipe courtesy of Waitrose