
Heat the stock. Bring the seafood stock and saffron threads to a boil in a medium saucepan over medium heat. Reduce the heat to low and keep the stock at a simmer. While the stock heats, begin working on the paella.
Sear the shrimp. Heat the olive oil in a large paella pan or a large heavy-bottomed pan over medium-high heat. Season the shrimp generously with salt and pepper. Add the shrimp to the pan and cook until seared, about 1 minute per side. Remove to a plate and set aside.
Brown the chicken. Season the chicken generously with salt and pepper. Add the chicken and cook, tossing occasionally, until golden brown, about 5 minutes. It does not need to be cooked all the way through at this stage. Transfer to a plate and set aside.
Cook the vegetables and calamari. Add the onion and squid. Cook, stirring occasionally with a wooden spoon, until the onion starts to brown, about 7 minutes. Add the peas and green beans, and cook, stirring, for 2 minutes. Add the tomatoes and garlic, and cook until the tomato reduces by half, about 2 minutes.
Cook the rice. Add the rice, mussels, clams, paprika, and reserved chicken pieces. Cook for 1 minute, stirring to coat the rice. Use the wooden spoon to distribute the contents of the pan into an even layer. Pour in the hot stock and add a pinch of salt. Cook, without stirring, for 5 minutes. Reduce the heat to low and let it simmer for 10 to 15 minutes, or until all of the stock has been absorbed and the rice is tender. About 3 minutes before the paella is done cooking, add the reserved shrimp on top of the rice.
Rest. Take the pan off the heat. Let the paella stand for 3 minutes before serving.