Meanwhile boil and mash your potatoes with the milk, butter and seasoning. While this happens, melt 10 tablespoons of the butter in a pan, and when it’s starting to bubble, add the flour and stir until the mixture smells biscuity (this suggests the flour is cooked), but do not let the mixture change color. There must be no browning at all, so be cautious with the heat. Then add the warm fish milk, whisking (as long as it is not an aluminium pan) as you go to avoid lumps. Let the sauce thicken until it easily coats the back of your spoon and check the seasoning.