Bring a small pot of water to a boil, add the shrimp and poach for 2 minutes until pink and opaque, then immediately transfer to an ice bath to stop the cooking.
Once cooled, drain the shrimp and chop into bite-sized pieces, then place them in a large glass mixing bowl.
Finely dice the red onion, English cucumber, and Roma tomato, then add them to the bowl with the shrimp.
Mince the jalapeño and finely chop the fresh cilantro, adding both to the mixture.
In a small jar, whisk together the lime juice, extra virgin olive oil, sea salt, and black pepper until emulsified.
Pour the dressing over the shrimp and vegetables, tossing gently to ensure everything is well coated.
Cover and refrigerate for at least 20 minutes to allow the flavors to meld and the shrimp to fully absorb the citrus.
Just before serving, dice the avocado and gently fold it into the ceviche to maintain its creamy texture without mashing.