In a medium non-reactive glass bowl, combine the snapper, lime juice, red onion, salt, and ground black pepper. The lime juice should cover the entire fish. Cover the bowl and refrigerate for 45 minutes to 1 hour, or until the fish turns opaque and becomes firmer. 

You can marinate for up to 2 hours for a whiter color, but keep in mind that the longer it sits in the acidic mixture, the firmer it will become.

In a medium bowl, mash the avocados, lime juice, and salt until chunky. Cover the bowl with plastic wrap, pressing the wrap directly onto the surface of the avocado mixture to prevent air exposure. Set aside temporarily. 

Once the fish is ready, remove it from the refrigerator and strain the lime juice. Transfer the marinated fish and onion to a large, non-reactive glass bowl. Add the habanero, mango, cucumber, tomatoes, red onion, and cilantro. 

Pour half of the reserved strained lime juice over the ceviche ingredients. Gently mix everything together until all the ingredients are well combined. Taste and adjust the salt and ground black pepper. 

Assemble the tostadas by spreading a layer of the creamy avocado mix on each one. Spoon the fresh ceviche over the avocado, and add a few drops of your favorite hot sauce for an extra kick of flavor, if desired.