In a large, nonreactive bowl, mix fish with lime juice and stir several times. Let sit, stirring often, until mostly opaque, about 15 minutes.
Add remaining ceviche ingredients and mix well. Season with more lime juice and salt if you like (salt balances the sourness of the lime).
Spoon ceviche onto tostada shells with a bit of juice, then top each with 3 slices avocado and a grind or two of pepper.
*To remove pin bones, which run lengthwise along fillet, grasp with tweezers (preferably fish tweezers) and pull out in the same direction that bones are pointed.