Preheat the oven to 180°C. Heat the butter and oil in a large pan. Add the leeks and cook over a low heat until soft.
Stir in the flour, then whisk in the milk and cream. Pour the mixture into a medium-sized baking dish. Top with the frozen haddock and mussels and season.
Spoon over the mash and spread using a fork, then bake for 25 minutes until golden brown.
Cook's note: Add extra smokiness to this old favourite by using smoked mussels from your pantry stash.