You can test for the done-ness of the plantain by sticking a fork or the tip of a knife into it. If it sinks into the plantain with little resistance, the plantain is done.
Depending on how ripe your plantains are, they may need up to 10 - 15 minutes to get done.
You can use any white fish for this dish. I used cod, but you can use Tilapia, Halibut, Sole or Haddock