Fish: Dice in ½ inch pieces, put fish in a large glass or plastic bowl and pour the lime juice over (making sure all fish is submerged). Cover and refrigerate for about four hours or until the fish has been cooked in the lime juice. Check to make sure fish is cooked by cutting a piece of the fish and making sure it is an even color in the center. Strain.
Vegetables: In a large bowl mix: tomatoes, onions, cilantro and Chiles Serranos. Add the cooked fish, olive oil and salt to taste. Mix all together. Cover bowl and refrigerate for approximately one hour for the flavors to marry. Keep the fish refrigerated until it is ready to be served.
To serve: Scoop the fish into glass bowls or small cups. Garnish with slices of avocado and sprigs of cilantro.