Heat olive oil in a paella pan over medium high heat. Add a little minced garlic and cook sliced chorizo for 2 minutes. Remove chorizo from pan. Add a little more olive oil and allow to heat. Add chicken and cook for 3 minutes. Remove from pan and set aside with chorizo. Heat more oil, add prawns and cook until shells are pink (about 2 minutes). Remove from pan and add more olive oil. When oil is hot add halibut and cook for about 2 minutes. Remove from pan and repeat process with squid. Remove and set all cooked ingredients aside.
Add another few tablespoons of oil to your seasoned paella pan and sauté onions until transluscent. Add rice and stir, coating each grain with oil. Cover with chicken stock and bring the liquid to a boil. Add a good pinch of saffron and continue to boil stock fairly rapidly on top of the stove until all the liquid has been absorbed and the rice is tender, 25 to 30 minutes. Stir from time to time, especially when the paella starts cooking, and add more stock or water if the liquid evaporates before the rice is cooked.
Ten minutes before the end of cooking, set the chorizo, chicken, prawns, halibut, squid and mussels on top of the paella and do not continue to stir it. Cover with a lid and cook for a further 5 minutes. Add tomato concasse, turn down heat and keep paella warm over a very low heat for 5 to 10 minutes before serving so that the flavours mellow.