For the onions: Place onion in a small bowl and sprinkle with salt. Add the vinegar. Then add just enough water to cover the onions. Set aside.

For the shrimp: Slice shrimp in half lengthwise (it's helpful to have the shrimp very cold when doing this), removing any remaining veins and/or tails. Place in a low wide glass bowl, in an even layer, and sprinkle with salt. Then pour lime juice over the shrimp. Fold gently to mix the lime juice well with the shrimp, as the lime juice will "cook" the shrimp with its natural acidity. Place in refrigerator to keep cold - and fold shrimp every 5 minutes or so, until all shrimp have turned from opaque to white/pink in color. This should take 20-30 minutes total.

For the marinade: Place all marinade ingredients in a high-powered blender and blend until completely smooth, scraping down sides as needed.

Add marinade to shrimp and fold to coat. Refrigerate for 30 minutes, and up to 4 hours.

To serve: Transfer marinated shrimp to a low wide serving bowl or platter. Drain the marinated onion slices. Top the shrimp with a scattering of onion, cucumber, and radish slices, plus offer additional on the side if desired. For those who really like heat, also offer additional slices of jalapeno. Serve cold with your favorite tortilla chips, saltine crackers, or mini tortillas. Avocado is also great with this!