To make the salsa, place the cucumber, tomato, red onion, chilli, mint and coriander in a bowl. Add the rice vinegar and soya sauce and mix well to coat, then season. Season, to taste, with salt and freshly ground pepper.
Meanwhile, heat a large non-stick frying pan. When hot, add the butter and put the fish fillets skin-side down on the pan. Season with salt and freshly ground black pepper abd cook for 2-3 minutes, or until golden and crisp.
Carefully turn the fish fillets and add the prawns to the pan. Cook for further 2-3 minutes, or until the fish and prawns are cooked through. Remove from pan and leave to rest in a warm place.
Tip the spring onions and mangetout into the pan used for the fish, cook for 2-3 minutes or until just tender.
To serve, divide the salsa between four plates, top with the spring onions and mangetout and place the fish on top skin-side up. Arrange the prawns around the plates and squeeze over a little lime juice.
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