Slice the tilapia into very thin slices. You could also do small dice and transfer to a bowl.
Add the line juice, orange juice, green onion and jalapeño.
Stir the mixture making sure to coat all the fish with the citrus juice.
Cover the bowl with plastic wrap and refrigerate for at least three hours, stirring once or twice in that time.
After three hours the fish should have turned white, if it's still a little opaque let it rest for another half hour.
Add the mango, tomato, cilantro and avocado and gently fold through with a wooden spoon.
Taste and add more salt and pepper if desired.
Transfer to a small serving bowl and serve with crunchy tortilla chips.