Preheat the oven to 400 degrees F.
Peel the plantains, rub the butter/oil all over the plantains and sprinkle salt (optional) over the plantains.
Roast the plantains in the preheated oven. You can place the plantains on the baking rack or over the lined baking sheet with foil and (also grease) then place the plantains.
Roast for about 20 minutes or until golden brown. You can flip/turn and bake the other side of the plantains but mostly I broil for about 7 minutes until it's golden brown. You can turn the plantains and roast for about 8-10 minutes.
When the plantains are all roasted, let it cool down. Slit down roasted plantains
Stuff the plantains with the caramelized onions and baked fish.