Heat two tablespoons of the oil in a 36cm/14in paella pan or a very large frying pan set over a medium heat. Season the chicken thighs with salt and freshly ground black pepper and fry for five minutes, turning regularly until lightly coloured. Add the chorizo and cook for a further minute. Transfer to a large heatproof bowl with a slotted spoon.
Add the onions to the pan and fry gently for five minutes until soft and lightly golden-brown. Add the remaining oil, red pepper and green beans to the onions and cook for two minutes, or until beginning to soften. Stir in the garlic and smoked paprika and cook for a further minute.
Stir in the saffron, bay leaf and rice and cook for one minute until the rice is coated all over with the oil. Return the chicken and chorizo to the pan, along with any juices that have collected underneath the meat. Stir well, then pour over the chicken stock. Bring to a simmer over a medium heat and cook for 12 minutes, stirring occasionally.
Put the mussels into the pan, making sure they are well tucked into the hot rice and steaming liquid. Return to a simmer over a medium and cook for five minutes until the mussels open.
Reduce the heat slightly, scatter the prawns over the top of the paella and continue cooking for a further 6-8 minutes, or until the rice is tender and almost all the liquid has been absorbed, turning the prawns once and pushing into the rice where you can. The prawns should be completely pink once cooked. It is important not to stir the paella after the mussels and prawns are added as you want the rice to become lightly browned and a bit sticky on the bottom as this adds lots of flavour.
Sprinkle with freshly chopped parsley and serve with the lemon wedges.