Heat 1 tablespoon olive oil in a large flat pan. Fry the chicken until slightly brown, remove and set aside. Wipe the pan clean with some paper towels.
Add 2 tablespoon olive oil to the same pan and fry the prawns, you may need to do this in two batches. Add the remaining 1 tablespoon oil if required. Fry the prawns for about 30 seconds per side. Remove from pan and set aside. Do not clean the pan
To the same pan add the onion, red pepper and garlic. Add a little more oil, only if required. Fry for a minute
Add the chopped tomatoes and paprika. Fry for a few seconds. Toast the saffron slightly in a clean pan, smash it using a mortar and pestle. Add 2 tablespoons warm water to the saffron and add it to the pan. Return the chicken to the pan. Sprinkle the rice in a thin layer. Pour the chicken stock (3&1/2 cups) into the pan. Ensure everything is immersed in the liquid, do not mix. You can add the remaining 1/2 cup of chicken stock only if required.
Cook on high heat until the liquid comes to a boil. Reduce to a medium heat and allow the dish to simmer for 20 minutes. Do not cover the pan. You can do a taste test to see if the rice is cooked, it should be firm but you will be able to taste if it is cooked.
Add the prawns and mussels and cook for a further 5 minutes. Turn the heat up and cook on a high heat for another 2-3 minutes. This is how the socarrat is formed but be careful not to burn the rice. Scatter pieces of butter on top (not necessary but highly recommended)
Once the dish is fully cooked it will still be moist but the rice shouldn't be stodgy. Cover the pan for 10 minutes and serve with fresh parsley and lemon slices