When water is boiling (enjoy the smell of the rosemary), place the fish pieces in the basket in a single layer (no overlapping), and cover. If necessary, cook the fish in batches. It's important that the fish is not in direct contact with water, only with the steam.
Cook for 4 or 5 minutes maximum. The fish is cooked when it is completely opaque. You can also use an electronic thermometer, and stop cooking when the middle of the fish reaches 60°C (140°F).