Place the flour, salt and pepper on a plate, mix well and use to coat the fish, shaking off any excess flour. Melt the butter in large frying pan until sizzling, add the fish and cook for 6 minutes on each side until well browned all over. Remove from the pan with a slotted spoon and set aside in warm place.
Meanwhile, place the mango, garlic, tarragon, onion, tomato, ginger, sugar and stock in a food processor and process until smooth.
Add blended sauce to frying pan, bring to the boil stirring then reduce the heat and simmer, uncovered, for 2 minutes.
Return the fish to the pan, spooning the sauce over fish, cover and simmer gently for 15 minutes, turning the fish once. Serve immediately.