Peel the sweet potato and cut into chunks. Add to a small saucepan of gently boiling water. Simmer for 10 mins until tender.
Meanwhile, in a saucepan, stir the Dijon mustard into the milk and heat gently. Add the cod fillet (make sure you check for bones first) and poach it for 6-8 mins until cooked through. Take off the heat, remove the cod with a slotted spoon and flake into small chunks, again checking for bones.
Mash the sweet potato with 3-4 tbsp of the poaching milk, then gently stir in the cod and the parsley.