In a medium bowl, combine the shrimp, onion, and 1/4 cup sun-dried tomato.
Measure the lime juice into a food processor or blender. Cover, turn on and drop in the chiles. When chopped, turn the blender off and scoop in the 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 mini tostadas and decorate with cilantro and diced sun-dried tomatoes.