Heat 0.5 tbsp of the olive oil in a 30 to 34cm paella pan or large shallow casserole dish over a medium high heat. Season the chicken on both sides then add them to the pan, skin side down. Cook for 5 minutes without moving them, then turn them over and cook for a further 3 minutes.
Once browned, transfer the chicken to a plate and set aside. Add the remaining 1.5 tbsp olive oil to the pan, turn down the heat to medium low, then fry the onion and beans with a pinch of salt for 8 minutes, stirring occasionally, until softened
Stir in the toasted saffron and both types of pimentón and cook for 1 minute until fragrant, then stir in the tomatoes and garlic. Pour in the chicken stock. Increase the heat to bring it to the boil, then sprinkle over the rice in an even layer, using a wooden spoon or spatula to stir it into the liquid. Stir only once - you're aiming to create a crust on the bottom of the pan (see Notes). Poke the chicken pieces into the rice along with the herbs, bring back up to a fast simmer, then bubble over a medium heat, uncovered, for 10 minutes.
Turn the heat down to low, simmer for another 5 minutes, then add the prawns in the spaces between the chicken pieces and cover the pan with a double layer of tin foil, tucking in the edges to seal the steam inside. Cook for a further 5 minutes, then take the pan off the heat and set aside, still covered, for 5 minutes to let the rice steam.
Sprinkle over the chopped parsley, then serve the paella with lemon wedges for squeezing, some crusty bread and a nice glass...or 2...of Spanish red wine!!