In a small bowl, combine the chopped red onion, fresh lime juice, olive oil, kosher salt, and black pepper.
Let the mixture sit for at least 5 minutes to soften the onion and develop the dressing.
In a large mixing bowl, combine the cooked chopped shrimp, diced tomato, diced avocado, and finely chopped jalapeño.
Pour the lime dressing over the salad ingredients.
Add the chopped fresh cilantro.
Gently toss everything together until evenly coated, being careful not to mash the avocado.
Taste and adjust the seasoning with additional salt, pepper, or lime juice if desired.
Serve the Shrimp and Avocado Salad immediately or chill for 15 minutes before serving for even more flavor.