In a medium bowl, combine the shrimp, onion and tomato. Measure the lime juice into a food processor or blender. Cover and turn on. Drop in the chile(s) and, when chopped, turn off and scoop in the avocado and cilantro. Blend until smooth. Thin to a "creamy dressing" consistence with water (usually 2 to 3 tablespoons). Taste and season with salt, usually about 1 teaspoon. (You will have about 1 1/2 cups.)
Mix the dressing into the shrimp mixture. Cover with plastic wrap directly on the surface of the shrimp a la mexicana and refrigerate. When you're ready to serve, scoop into a serving bowl, decorate with a cilantro sprigs and it's ready.