Cut the fillets in half lengthwise, then slice each half crosswise into thin strips and place in a medium-size nonreactive bowl. Add the orange, lemon, and lime segments and the orange, lemon, and lime juices and stir to combine. Add the onion, avocado, coriander and chilli, and gently fold to combine.
Refrigerate the mixture for 1 hour for well-done fish or serve immediately for fresh tartare, sushi-like texture.
Serve cold in small bowls, with tostones, if using, if desired.