- Chop the onion fine and sauté it in the butter for about 10 minutes. Make sure it doesn't go brown.
- 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 and add the vinegar.