Potato and green bean salad

Name: Potato and green bean salad
Ingredients: 750g/1lb 10oz baby new potatoes, halved; small bunch fresh mint, leaves picked and roughly chopped; 2 small bunches fresh flatleaf parsley, roughly chopped; 60g/2¼oz capers, roughly chopped, plus 3 tbsp brine from the jar; 50g/1¾oz cornichons, roughly chopped; 4 pickled guindilla chillies, roughly chopped; 1 lemon, juice only; 1 tbsp Dijon mustard; 180ml/6fl oz olive oil; 4 spring onions, finely sliced; 200g/7oz fine green beans, halved; salt and freshly ground black pepper