- Melt the butter in a clean, smooth pan.
- Add the flour and mix the flour and butter to a white mass.
- Then add the boiling water or fish stock in small amounts, stirring well and in between bringing the sauce back to the boil.
- Then leave to boil for 10 minutes and season with salt.
- Take the sauce off the heat add part of the warm sauce to the beaten egg in small amounts.
- Once well beaten, add this back to the rest of the sauce and then whilst beating add the lemon juice in.