Prepare the shrimp by boiling in salted water for 2-3 minutes until pink, then immerse in ice water.
Chop the shrimp into bite-sized pieces and place in a mixing bowl.
Pour fresh lime juice over the shrimp and let marinate for 15 minutes.
Add finely chopped red onion, cucumber, tomato, and jalapeño to the bowl.
Toss gently to combine all ingredients.
Add chopped cilantro, salt, and black pepper to taste.
Mix carefully to avoid breaking the shrimp.
Let sit for an additional 10 minutes to meld flavors.
Serve chilled, garnished with avocado slices.