In a medium bowl, combine the shrimp, onion, and ½ of the tomatoes.
Measure the lime juice into a food processor or blender along with the chiles, avocado and cilantro. Process until smooth. Thin to a "creamy dressing" consistency with water, usually 2 to 3 tablespoons. Taste and season with salt, usually about 1 teaspoon. (You will have about 1 1/2 cups of dressing.) Mix the dressing into the shrimp mixture. Cover with plastic wrap directly on the surface of the shrimp and refrigerate. When you're ready to serve, scoop onto the tostadas and decorate with cilantro and the remaining tomato.