Drain the water from the hearts of palm can. Slice them in half lengthwise and slice them ¼ inch thick.
Peel the cucumber, cut lengthwise, and use a spoon to scoop out the seeds. Do not discard the seeds and flesh.
In a large bowl combine the hearts of palm, 1 cup of the sliced cucumber, and the sliced red onion.
In the blender, combine the serrano pepper, lime juice, chopped cilantro, the additional 1 cup of sliced cucumber, the seeds and flesh that was scooped out of the cucumbers, and process until smooth.
Pour this liquid over the hearts of palm mixture, season with salt and pepper, and let sit in refrigeration for 10 min.
Adjust seasoning and serve on crispy tostadas with sliced of avocado.