Thinly slice onion and leeks. Grate fennel and celery.
Heat olive oil in a large saucepan and saute vegetables over medium heat for 10-15 minutes,or until they are soft and translucent.
Add saffron and tomato paste and stir well.
Add pastis and vermouth,using them to deglaze the pan.
Add chicken and fish stocks and bring to the boil. Reduce heat to low and simmer for 30-40 minutes.
Check taste and season accordingly.
Put the clean mussels in a large pan which has a lid. Add a splash of pastis,put the lid on and steam mussels over medium heat until the shells open.
Cut rock cod fillets into pieces that are approximately 80g each.
Return bouillabaisse to the boil,add rock cod and lobster tails. Reduce heat and simmer for about 5 minutes,or until the seafood is cooked.
Remove seeds from tomatoes,dice into 5mm cubes and add to the bouillabaisse.
Add parsley and a splash of Pastis for an extra kick.