Wash the fish in water and lemon or lime juice. If the fish has scales remove them by running a knife against the grain of the scales.
Bring the water (2 litres) to boil, add the fish then simmer for 30 minutes.
Chop the banana, potato, garlic and scallion.
After half an hour of simmering, strain the fish out of the water.
Add the green banana, potato, scallion, garlic, pepper (unchopped) salt and black pepper to the fish stock.
When the fish has cooled, separate the bones from the flesh.
Add the fish to the stock and simmer for 30 minutes.
Serve in a bowl.