Boil the potatoes, put them through a mouli or ricer and into a bowl.
Proceed as above for the cod till you reach the processor stage. Whizz. But add only a very little oil. Remove from the processor and put into the bowl with the potatoes, crushed garlic and chopped parsley. Mix together, adding oil only to ease the homogenisation. The aim is to achieve a stiff paste that can be formed