In a large bowl, toss shrimp with 1 teaspoon kosher salt and baking soda until evenly coated. Transfer to refrigerator for 15 minutes.
Meanwhile, in a medium bowl, stir together 4 tablespoons lime juice with onion, tomato purée, ketchup, cilantro, orange juice, and jalapeño.
In a medium pot, combine 2 quarts cold water with remaining 2 tablespoons lime juice and 2 tablespoons salt. Add shrimp, set over medium-high heat, and cook, stirring occasionally, until temperature reaches 170°F (77°C) on an instant-read thermometer and shrimp are just cooked through; adjust heat to make sure temperature does not go over 170°F.
Drain shrimp carefully, rinse under cold running water, then drain well. Cut shrimp into ½-inch pieces. Add shrimp to bowl with sauce and toss to combine. Taste, then add more lime juice if desired.
Top with avocado and serve immediately with saltines and hot sauce.
Serious Eats, shrimp, Mexican,