just bring a medium pot salted water to a boil (a Tbsp salt for 2 quarts of water), add the shrimp, and cook for 2-3 minutes, until just cooked through. Reserve a cup of the cooking water and chill it. Cut half of the shrimp into large chunks, leaving the other half whole. Put the chopped shrimp in a bowl with the chopped onions, cucumber, celery, jalapeño, tomatoes, ketchup, cilantro, lime juice and hot sauce. Gently stir to combine, and chill. Once the cooking water has cooled at least to room temperature, add a little at a time to the shrimp cocktail, until the shrimp cocktail is the consistency you want (use clam stock or lightly salted water—a third of a teaspoon of salt dissolved into a cup of water—if you are starting with already cooked and peeled shrimp).

   To serve, add salt and more hot sauce to taste and gently mix in the chopped avocados. Dip the whole shrimp in the cocktail and arrange them on top.