Preheat the oven to 200'C.
Do not de-scale the fish (this is very important as without scales the salt will penetrate through the fish too deeply).
Cut the fennel, one lemon and onion into quarters. Gut the fish and fill the belly cavity with the quartered ingredients. Remove the eyes and gills.
Blitz the fennel seeds in a food processor and then add to the zest of lemon and the 2 salts. Combine with the water to make the salt crust. Press the crust on to the surface of the fish - it should be compact and around half a centimetre thick all over.
Make a platform out of tin foil by folding it over 10 times to create a firm base. The two long edges in to give a rough shape of the fish. Place the fish on top and cook in the oven for 28 minutes.
Allow to rest for 5 minutes before cracking the salt crust with a spoon. Carefully left off of the salt, allowing it to take the skin with it.
Serve the sea bass with a simple salad and a wedge of lemon.
«Take care to cover the fish with an even layer of salt.»