Half the oil in a pan, and then add the onion cut in very small pieces and slightly fry it (about 5 minutes). Next add the peeled and deseeded tomatoes, cut into pieces. Lightly fry for another 5 minutes, crushing the tomatoes with the edge of a palette knife. When ready, strain (smash) and pour into the “paellera” (Paella-pan).
In a separate pot, place only the fish bones (keeping the fish apart) and the prawn shells. Add cold water, bring it to boil.
Wash the cockles and mussels well in water and salt, place them in another pot and bring to the boil. The moment the shells are opened, remove the pot from the heat and remove the empty shells without bodies inside, keeping the ones with the animal.
Take both boiling liquids and pour through a fine sieve.
Add the remaining oil to the paellera where rice is to be cooked (remember there is something already in there). Place the paellera on a low and even heat. If using green pepper, cut it in squares of 3 cm. Add to the paellera and fry lightly. Then add the squid previously cut in long strips of 1/2 cm. wide and 4 cm. long, or in rings. Next add the fish that should be cut in small pieces and finally add the rice to the pan.
Stir with a wooden spoon and take care not to let it brown. Add some salt, and finally, add the sieved liquid, which has to be hot but not boiling. If necessary add some hot water to make up to the 5 mugs (that is the amount of liquid necessary for the rice to be cooked).
Shake the paellera but only by the handle so the fluid is well distributed.
Meanwhile in a mortar, pound a little garlic, the parsley and the saffron with a pestle, together with a pinch of salt to avoid slipping, and at the end, add a couple of tbsp. of warm water.
Pour the mixture over the rice and shake the paellera again by the handles or spread the contents with a spoon all over the surface. Now add the bodies of the prawns, and when the liquid is half cooked, add the red pepper in strips, the cockles and mussels, the green peas and the red sausage.
It normally takes about 20 minutes from the moment you add the liquid, depending on the sort of rice used.
Once the rice is tender and the liquid absorbed, remove the paellera from the heat, place on a wet cloth, leaving it to rest for about 5 minutes.
For an attractive appearance serve with lemon slices arranged around the border, or squeeze a few drops of lemon into the paella.
Some people prefer to use both lemon slices and squeezed lemon, others add the lemon juice to the liquid before cooking. They say this prevents the rice from becoming sticky.