Place the chopped shrimp in a large bowl and pour the lime juice and lemon juice over it. Stir to ensure the shrimp is fully submerged in the citrus juices.

Cover the bowl with plastic wrap and refrigerate for 30-40 minutes, stirring occasionally, until the shrimp turns opaque and is fully 'cooked' by the acidity of the juices.

While the shrimp marinates, prepare the other ingredients: dice the red onion, tomato, cucumber, jalapeño, and chop the cilantro.

Once the shrimp is ready, drain any excess liquid and return the shrimp to the bowl.

Add the red onion, tomato, cucumber, jalapeño, and cilantro to the shrimp. Mix well to combine.

Season the ceviche mixture with salt and black pepper to taste.

To assemble, place a generous amount of shrimp ceviche onto a tostada shell.

Top with sliced or diced avocado and serve immediately.