This recipe is ideal for a dinner party because so much can be done in advance. The mullet may be prepared up to 1 hour ahead, covered and kept in a cool place. The rouille can be made up to 24 hours in advance, covered and kept chilled. However, if you do make it ahead, use only one chilli because the flavour is absorbed more by the mayonnaise over a longer period and it will become very fiery indeed.