On the eve of the cod soak in cold water to remove excess salt. Wash the potatoes and boil it in salted water brown 20 minutes Drain, peel the potatoes from the skins and mash with a fork. Remove from the water cod and cut it into small pieces.
In a large pot of water, pour the milk, add the bouquet of herbs, a few grains of pepper and bring to a boil. Reduce heat, put in water cod and cook it for 8 minutes, then drain the water, chop the fish and put it to cool. Peel garlic and squeeze. Pour into a large pot of sour cream and olive oil, then add the garlic and pepper. Bring to a boil, then add the chopped potatoes and cod. Mix everything and cook over low heat until a homogeneous mixture, then remove from heat. Put the mixture into the baking pan and cook in the oven until golden brown. Sprinkle with olives and chopped chervil.