Bring a pot of water to a boil and poach the shrimp for 2 to 3 minutes until they are pink and opaque.
Immediately transfer the shrimp to an ice bath to stop the cooking process, then peel, devein, and chop them into half-inch pieces.
In a large glass mixing bowl, whisk together the fresh lime juice, extra virgin olive oil, sea salt, and black pepper to create the marinade base.
Finely dice the red onion, Roma tomato, and jalapeño, ensuring you remove the jalapeño seeds if you prefer a milder flavor.
Roughly chop the fresh cilantro and add it to the bowl along with the diced vegetables and the prepared shrimp.
Toss the mixture thoroughly so the lime juice begins to penetrate the shrimp, then cover and refrigerate for 30 minutes.
Just before serving, dice the avocado and gently fold it into the ceviche to keep the pieces intact and creamy.