Remove prawn heads, shells and devein, and set flesh aside. Place the stock, fish frame, cleaned prawn heads and shells into a large pot. Simmer the mixture over MED heat for 25 minutes, and then strain and set the stock aside.
Submerge the tomatoes in boiled water for 5 minutes. Then remove from water, peel the skins, crush and finely chop the tomatoes.
Soak saffron in 1 tablespoon of hot water and set aside.
Heat 1 tablespoon of oil in the pan over HI heat. Add the diced onion and sauté until it becomes translucent. Add the garlic, tomatoes, peppers and spice and cook while stirring for 3 minutes.
Turn the heat to LOW. Add the rice and gently stir, spreading the rice evenly in the pan.
Add 400ml of the stock, and the saffron and water mix. Bring to the boil, then simmer on MED heat for 10 minutes until most of the stock has been absorbed. Do not stir, but do rotate the pan to ensure it cooks evenly.
Arrange the seafood on top of rice. Turn the heat to LOW-MED, and cover pan with foil and cook until the prawns are opaque (approximately 5 minutes) and the mussels have opened. Turn the prawns over halfway through cooking. Add more stock if it is needed.
Scatter the peas over the top, cover again and then cook for 2 more minutes to help achieve a crispy base.
Remove from heat but leave covered and allow to rest for 5-10 minutes.