Preheat your oven to 450°F and lightly oil a sheet pan with vegetable oil.
Arrange the shrimp in a single layer on the sheet pan and sprinkle Tajín over them. Give the shrimp a quick toss to coat them and roast them for 8 minutes, or until pink and curled.After the cooking time has elapsed, remove them from the oven and allow them to come to room temperature on the counter.
In a large mixing bowl, combine the diced celery, cucumber, the minced red onion, garlic, cilantro, jalapeño, avocado, and lime juice. and add that to your bowl, as well.
In a separate bowl, whisk together the ketchup, horseradish, hot sauce (if you're using it), and Clamato juice.
Gently fold the cool shrimp into the bowl of veggies, then add the tomato sauce. Season the mixture with the salt. Refrigerate the cocktail for 30 minutes before serving, or until ice cold.