First make the mango miso dressing. Put the mango puree in a medium bowl with the lime juice, miso and half a teaspoon of salt, and mix well. Pour in the oil in a slow, steady stream, whisking continuously, until you have a smooth dressing, then set aside.
Put the chopped tomatoes and mango in a medium bowl with 80ml of the dressing and three-quarters of a teaspoon of salt, mix gently, then leave to marinate for 20-30 minutes. Drain into a colander and discard the liquid.
Meanwhile, for the herb dressing, roughly chop the basil, mint and chilli, and put them in a small bowl with the oil, a quarter-teaspoon of salt and a good grind of black pepper.
Using a small, sharp knife, top and tail the remaining lime, then cut around its natural curves to remove the skin and pith. Cut between the membranes to release the individual segments, then cut each segment into four or five pieces, add to the herb bowl and mix well.
Arrange half the onions on a medium platter, followed by the drained mangoes and tomatoes, then spoon over the remaining mango dressing. Dollop the herb dressing on top and scatter over the remaining onions. Serve with the nigella seeds sprinkled on top.
