Method

1. Put the water, lemon and salt in a large pan. Bring to the boil, then simmer for 5 minutes.
2. Add the hake and simmer for 2 minutes. Remove from the heat and leave to finish cooking gently in the cooking liquid.
3. Melt 25g/1oz butter in a heavy-based pan and sprinkle over the flour. Stir continuously with a wooden spoon to mix, then cook until the mixture smells nutty.
4. Gradually pour in the milk, stirring all the time, to make a smooth sauce. Add 290ml/½ pint of the fish cooking liquid and leave to simmer for 12 minutes.
5. Drain the hake and cut into 4 portions. Place on four warmed plates. Stir the chopped parsley and the rest of the butter into the sauce.
6. Pour the sauce over half the fish. I think dishes like this look more attractive if the fish is only partly covered. Serve with some potatoes boiled in salted water and a sprig of mint.