Heat the oil in a large frying pan. Add the onion, garlic and red pepper to the pan and fry for a few minutes until they have started to soften, then add the rice and fry for 1 minute.
Pour enough hot stock over the rice to cover it by about 1 cm (½ inch). Add the saffron threads and stir well. Bring the rice up to the boil, then add the tomatoes and reduce the heat to a simmer. Stir well once again, then simmer for 10–12 minutes, stirring occasionally to prevent the rice catching on the bottom of the pan.
Add the prawns, clams and mussels (first discarding any that don't shut when tapped) and squid to the pan, along with a little more water or stock if the rice is too dry. Cook until the clams and mussels open (discarding any that don't), the prawns are pink and the squid turns white and loses its transparency.
Stir in the peas and parsley and cook for a few more minutes until the peas are hot, then season to taste with salt and pepper.