2 Steep the saffron: In a saucepan over medium heat, bring the stock to a boil. Add the saffron and salt. Turn off the heat and let the saffron steep for at least 15 minutes. Taste and add more salt, if needed.
3 Cook the sofrito base: In a 12- to 14-inch stainless steel skillet or cast iron pan, heat the oil over medium heat on top of the stove. Add the onion and red pepper, and cook for 5 to 7 minutes, or until the onion is translucent. Stir in the garlic and chorizo.
4 Assemble the ingredients by the grill: On a table next to the grill, set the skillet with the sofrito, the rice, tomatoes, infused stock, salt, peas, prawns, mussels.
5 Begin cooking the paella: Set the skillet with the sofrito on the grill. Add the rice, and cook, stirring often, for 4 to 5 minutes, or until the rice is coated with oil and lightly toasted.
Stir in the stock, tomatoes, and peas. Taste for seasoning and add more salt, if you like.
Spread the rice evenly over the bottom of the pan. Close the grill cover and simmer the rice without stirring for 15 minutes, or until the rice absorbs most of the stock. If the mixture looks dry, pour about 1 cup of hot water over it, but do not stir.
6 Add the seafood: Nestle the mussels into the rice with the hinge sides up so they release their juices into the rice. Arrange the prawns around the shellfish.
Cover the pan with foil, close the grill and cook for 6 to 10 minutes longer (depending on the heat of your grill), or until the rice and prawns are both cooked through.
7 Check to see if the bottom is browned: Slip a spatula under the rice and check to see if you have achieved the elusive golden brown socarrat. If not, set the pan over the heat, uncovered, for a few minutes to lightly caramelize the bottom.
Sprinkle with parsley and bring the whole pan to the table for serving.