Cut the fish into a small dice. Place in a class bowl and pour the lime juice over the fish, using enough to completely cover it.
Refrigerate for several hours until the fish no longer looks raw, about 2-3 hours (no longer than 6 hours).
Drain off the lime juice using a colander and combine the fish with the remaining ingredients (except for the avocado) and season to taste with salt. Add the avocado just before serving.
Serve in a martini glass on a plate with the tortillas chips around the base.
Recipe by Culinary Thymes at https://www.culinarythymes.com/ceviche-tostadas/