POACHED TROUT WITH POTATO & PICKLED SALAD

½ red onion, finely sliced • 135g radishes, finely sliced • ½ cucumber, halved lengthways and sliced • 1 tbsp red wine vinegar • 1 tbsp roughly chopped mint leaves, plus some small leaves to serve • 340g baby new potatoes, halved if large • Zest and juice of ½ lemon • 2 English Trout Fillets (about 265g, from the fish counter) • 2 tsp Bart Fish Seasoning • ½ tbsp olive oil