Clean the fish belly, rinse it thoroughly and pat dry with kitchen paper. DO NOT REMOVE THE SCALES.
Lay some salt in the bottom of the oven dish to make a bed, I would say about 5 mm (3/16") thick.
Put the rosemary sprig into the belly.
Add few drops of olive oil inside the belly. You can also put in some finely chopped garlic but I suggest doing this the next time you cook this recipe and then you can taste the difference.
Close the fish belly so that when you cover it with salt, the salt will not go inside.
Cover with salt.
The fish should disappear under a layer of salt. Use just enough salt to cover the whole fish, no more no less. Pre-heat the oven at 200°C (400°F) and then put the oven dish in the oven for about 30 minutes.
As a rough guide, the fish is cooked when the salt makes a hard crust and the colour of the salt is not white anymore but a kind of light grey/brown. Also, a nice smell will suddenly spread through your kitchen.
Then remove the oven dish carefully from the oven because it will be very hot. Start breaking the crust around the fish shape with the point of a knife.
Do not go too deeply with the knife because we do not want to damage the fish. Then, gently try to make a small hole on the top and continue to enlarge the hole step by step until you can see that you can lift pieces of crust away.
There you go! You can see the crust and also you can see that the fish skin does not stick at all and protects the fish meat.
When you have freed the fish, put the fish onto a plate and discard the remaining salt.
Now, take the fish meat and with a knife remove the few salt grains you may find nearby. Put the meat in another dish as shown in the next picture.
Sprinkle with few drops of olive oil and serve it.