Cut 1 lb Corvina (or other firm white fish) into small cubes, about 1/2 an inch thick. Add to a large mixing bowl.
Dice 1/2 red onion, 1 avocado, 2 tomatoes, and 1/4 cup cilantro. Add to bowl with 1 tsp salt and combine.
Juice 4-5 limes directly into the bowl with all the ingredients. 4-5 limes should yield about 1/2 to 3/4 of a cup of lime juice.
Mix together and make sure the lime juice completely covers the fish. Allow the ceviche to sit in the lime juice for at least 15 minutes before serving.