Heat 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.
Add the garlic and cook for a further couple of minutes, then push everything to one side. Add the tomatoes and fry for a few minutes, then mix everything together and sprinkle in the paprika and lemon zest. Pour in all but a large ladleful of the chicken stock and return to the boil. Add the bay leaves, thyme and green beans and season with a little salt.
When the chicken stock has returned to the boil, turn down to a simmer and sprinkle in the rice, trying to get it into as even a layer as possible. Pour in the saffron and stir – this is the only time you should stir your paella once the rice has been added! Bring to the boil again, then simmer for 5 minutes.
Turn the heat down and leave the rice to simmer slowly – this should take between 12-15 minutes. If it is getting dry, taste – if still a little too firm (crunchy), add a little more stock. When you are satisfied the rice is al dente, arrange the mussels and prawns over the paella. Cover with a damp teacloth or a lid if you have one and wait for the mussels to steam open and the prawns to turn pink. Serve with lemon wedges.