Dice the fresh fish into small, bite-sized pieces.
Place the fish in a glass or ceramic bowl.
Pour fresh lime juice over the fish, ensuring it is fully submerged.
Add salt and pepper to taste, and stir gently.
Cover the bowl with plastic wrap and refrigerate for 15-30 minutes.
While the fish is marinating, prepare the vegetables.
Finely chop the red onion, tomatoes, cucumber, and cilantro.
Add the chopped vegetables and serrano peppers to the fish after marinating.
Mix everything gently to combine the flavors.
Serve chilled, garnished with avocado slices.