This is a simple version of escovitch fish (the Caribbean version of what Europeans call escabeche fish). You can use whatever vegetables you have to hand; they pickle nicely in the dressing as the fish cooks.
Preheat the oven to 190°C/375°F/gas mark 5. Lay each trout on a generous piece of foil (the foil needs to be big enough to make a parcel round the whole fish). Season the trout inside with salt and pepper and stuff in the spring onion to give lots of fresh flavo