Soak the saffron in a little warm water. Cook the chorizo in the paella pan or very large frying pan set over a medium heat for about 2 minutes or so. Preheat your chicken stock in a separate pan on the side and keep warm.
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 (if required, though the chicken stock will already be quite salty).
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 tea-cloth or a lid if you have one and wait for the mussels to steam open and the prawns to turn pink. Sprinkle over a small amount of lemon juice. Serve with lemon wedges.