Bring 5 cups water, ½ peeled large white onion, 5 peeled garlic cloves, and 1 teaspoon kosher salt to a boil in a large pot or Dutch oven over medium heat. Add 2 pounds raw extra-large unpeeled shrimp and cook until the shrimp are pink and just cooked through, 4 to 5 minutes.

Turn the heat off. Using a slotted spoon, transfer the shrimp to a large bowl and let cool until warm to the touch, about 15 minutes. Keep the cooking liquid in the pot.

Peel the shrimp but keep the tails on if desired; add the shells to the cooking liquid. Return the cooking liquid to a boil, then cook over medium heat for 5 minutes.

Pour through a strainer set over a heatproof bowl; discard the contents of the strainer. Reserve 1 cup of the shrimp stock and let cool to room temperature, about 20 minutes. (Discard or reserve the remaining shrimp stock for another use.) Meanwhile, clean the shrimp and prepare the remaining ingredients.

Remove the intestinal tracts from the shrimp if desired. Refrigerate shrimp until ready to use.

Finely dice the remaining ½ large white onion (about 1 cup). Place in a large bowl. Stir in 4 cups V8 Vegetable Juice, 1 cup ketchup, 1 small diced large cucumber, 1 small diced large tomato, ½ cup coarsely chopped fresh cilantro, 2 small diced medium jalapeños, 3 tablespoons lemon juice, and 1 teaspoon Maggi seasoning.

Stir in the reserved shrimp and cooled shrimp stock. Taste and season with more kosher salt and black pepper as needed. Refrigerate for at least 20 minutes or up to 1 day. Serve with Mexican hot sauce, diced avocados, and tostadas if desired.