Place the cod in a bowl with the milk and heat it in the microwave, just 2 or 3 minutes on high, until barely cooked. Drain the fish and keep the milk for later. Leave to cool, remove the skin and bones and flake it. Set aside.
Pour 6 tablespoons of the olive oil in a skillet and heat, add the fish and stir continuosly with a wooden spoon, mashing and mixing, until all the oil is absorbed. At this point you can use a hand food processor to make the purée smoother, add the Roasted garlic meat, the boiled potato.
From then on, start adding the rest of the olive oil (previously heated) and the reserved milk (also warm) little by little, one spoonful of olive oil per half spoonful of milk, alway stirring (or mixing on very low speed) and adding the next spoonful only when the previous one has been absorbed by the mixture, until all the milk and the olive are finished.