First soak the cod – you will need to start this process a full day before you want to make the brandade. Put the cod in a large container and cover with cold water. Leave to soak at room temperature for 24 hours, changing the water at least four times.
Drain the cod and put in a saucepan. Cover with water and bring to the boil. Simmer for five minutes, then drain. Repeat this process.
Return the cod to the saucepan and this time cover with the milk. Add the bay leaves, garlic cloves, peppercorns, fennel seeds and lemon zest. Heat gently until almost boiling, then turn down and leave to simmer for 10 minutes. Remove from the heat and leave the fish to cool in the milk.
Cut the potatoes into small chunks and put in a saucepan. Cover with water and bring to the boil. Simmer until tender – around 10 minutes – then drain.
Strain the cod, reserving 50ml/2fl oz of the milk, but discarding the aromatics. Remove any skin and bone from the cod, then crush the flakes lightly with a fork.
Make the sauce in a food processor or by hand. To make in a food processor, put the fish and potatoes in the processor with the reserved milk and the cream. Blend until becoming smooth, then gradually drizzle in the olive oil in while the motor is running until you have a smooth, well emulsified paste.
Alternatively, mash the cod with a fork, or pound with a pestle and mortar, gradually incorporating the reserved milk, then mash the potatoes with the cream. Mix together then gradually work in the olive oil.
Serve with a little finely grated nutmeg, a drizzle of olive oil and perhaps a few slices of truffle, along with toasted slices of French bread.