Slice the red onion thinly with the grain, and toss in a bowl with salt, pepper and lime juice, coating well.
Add the shrimp
Add the tomatoes, cucumber, cilantro and olive oil, and give a stir and marinate in the refrigerator for at least 30 minutes before serving (45-60 minutes is ideal). The longer you marinate the firmer and more “cooked” the fish will become.
Before serving, taste and adjust the salt and heat. Add more salt or chilies if you like. If adding avocado, gently fold it in right before serving- you may need to add a pinch more salt.