Boil some water in a pot. Clean the mussels carefully with some cold water and the help of a brush if they are too dirty and hairy, and throw them into the boiling water. Then, take them out as soon as they are opened. Shell them (you can keep some of them with the shell if you like) and set them aside.
Chop the spring onion, garlic and peppers into thin pieces. Clean and cut the squid into rings. Remove the heads of the prawns.
Heat two tablespoons of olive oil in a big pan and fry the heads and peels of the shrimps and the heads of the prawns until they start to brown. You can push them gently with a wooden spoon or something similar to release more of their juices. Remove them from the pan, making sure that you leave (almost) no traces of them.
Toss the chopped vegetables into the same pan and stir-fry them over medium heat until they are soft, stirring occasionally. Cut the tomatoes into medium-sized pieces of about 5 by 5 cm and add them to the pan. Mix everything together and keep cooking for about 10 minutes over medium heat. The tomatoes should release a lot of their juices, you can press them a bit with a wooden spoon to release as much juice as possible.
Add the squid or cuttlefish. Mix everything together well and cook for another two minutes.
Add the rice and some strands of saffron, mixing everything together well.
Pour the fumet or fish stock on top of everything.
Cook over high heat for 6 minutes. Then lower your heat down to the lowest setting for another 12 minutes, making sure that the stock keeps boiling. Keep in mind that around the 12-minute mark (4 minutes after you turned your heat down) you should add the beheaded prawns, as they will need around 8 minutes to cook. Add the mussels as well.
Turn your heat back on high for the last 2 minutes. Try some of the rice, adding some salt if necessary. Turn your heat off and let rest for 2 minutes before serving.