In a large bowl, combine chopped red onion, lime juice, olive oil, salt, and black pepper. Let it sit for about 5 minutes to soften the onion and build flavor.
In a separate bowl, gently mix the shrimp with diced avocado, tomato, and jalapeño.
Add the shrimp mixture into the bowl with the lime dressing.
Sprinkle chopped cilantro over the top and gently toss everything together, being careful not to mash the avocado.
Taste and adjust salt or pepper as needed.
Serve immediately for the freshest flavor and texture.