Heat the oil in a paella pan or a wide, deep, heavy-bottomed frying pan.
Add the chicken thighs and cook until browned on all sides. Add the pork cubes, squid and chorizo and fry until golden-brown all over, then remove from the pan and set aside.
In the same pan, fry the onion and garlic for 3-4 minutes, until softened.
Add the rice, rosemary, saffron and its milk, and about half of the stock. Bring to the boil, then reduce the heat to simmer for 18-20 minutes, adding more stock if the rice dries out.
Return the chicken and pork to the pan, stir well and allow continue simmering for ten minutes, adding a little more stock if necessary, until the chicken is completely cooked through.
Add the cooked squid and chorizo and stir well.
Add the prawns, assorted fish and the tomatoes and cook for about 8-10 minutes, or until the fish and prawns are just cooked through.
To serve, spoon the paella into warmed bowls with a lemon wedge alongside.