Dice the fresh fish into small uniform pieces.
Combine fish and lime juice in a glass bowl. Cover and refrigerate for 15-20 minutes.
Chop the red onion, tomatoes, cucumber, and jalapeño.
Drain excess lime juice if necessary.
Add chopped vegetables and cilantro to the fish and mix gently.
Season with salt and pepper to taste.
Serve immediately, garnished with diced avocado and lime wedges.