Ceviche de Camaron: Shrimp Ceviche Cocktail

Name: Ceviche de Camaron: Shrimp Ceviche Cocktail
Ingredients: 1/2 cup plus 2 tablespoons fresh lime juice; 1 generous pound unpeeled smallish shrimp (I prefer the ones that are 41/50 count to a pound); 1/2 medium white onion, chopped into 1/4 inch pieces; 1/3 cup chopped fresh cilantro, plus several sprigs for garnish; N/A fresh cilantro sprigs; 1/2 cup ketchup; 1 to 2 tablespoons vinegary Mexican bottled hot sauce (such as Tamazula, Valentina or Búfalo, the latter being on the sweet side); About 2 tablespoons olive oil, preferably extra-virgin (optional, but recommended to smooth out sharpness); 1 cup diced peeled cucumber or jícama (or 1/2 cup of each); 1 small ripe avocado, peeled, pitted and cubed; Salt; Several lime slices for garnish; Tostadas or tortilla chips, store-bought or homemade or saltine crackers for serving