1. Mix shrimp with teaspoon of kosher salt and baking soda. Refrigerate for 15 minutes.
2. Combine 4 tablespoons of lime juice with onion, tomatoes, cilantro, ketchup, orange juice and serrano pepper in a large bowl, then set aside.
3. Use a medium boiling pot and mix in 2 liters of cold water with 2 tablespoons of lime juice and 2 tablespoons of kosher salt. Add shrimp and boil over medium-high heat. Stir occasionally, until temperature reaches 170°F. Be sure to not let temperature go over 170°F.
4. Drain shrimp carefully and rinse under running cold water. Rinse thoroughly.
5. Add shrimp into mixture in large bowl and stir.
6. Serve in shrimp cocktail glasses and garnish with avocado slices. Enjoy with hot sauce and salted crackers.