In a large bowl, toss together the tomatoes, cucumber, avocado, shredded imitation crab, red onion, cilantro and sliced serrano.
Cook The Shrimp:
Set up a large bowl of ice and water. In a medium saucepan, bring 5 cups (1.2 liters) water to a boil over high heat. Add 1 pound (455 g) of the shrimp and cook until they turn completely pink, 3 to 4 minutes. Use a slotted spoon to immediately transfer the shrimp to the ice bath to stop them from cooking. (Letting them cook any longer will result in rubbery, overcooked shrimp.)
In a medium bowl, combine the remaining 1 pound (455 g) shrimp and the lime juice, making sure all of the shrimp are completely submerged. Cover the bowl with plastic wrap and place in the fridge. Let the shrimp “cook” in the lime juice until they are completely pink in color, 15 to 20 minutes. Drain the shrimp.
Make the Seasoning:
In a small bowl, whisk together the lime juice, celery salt, granulated garlic, and black pepper.
Make the Ceviche:
Fold the lime-cooked shrimp and the parboiled shrimp into the vegetables then pour the seasoning over the ceviche and toss everything together once more, then taste for salt and adjust.
Serve the Ceviche
Top a tostada with 1 cup (200 g) to 1½ cups (300 g) of the ceviche. Top the ceviche with avocado and finish with a sprinkle of sea salt. Serve with lime wedges and your favorite Mexican hot sauce.