Place the white wine and lemon juice into a medium sized shallow saucepan with the diced shallots and bring to the boil.
Reduce by about a half and then strain the liquid to remove the shallots
Reheat the mixture if it has cooled down a little.
Next whisk in the butter piece by piece, beating well between each addition until al of the butter has been incorporated into the sauce. I find the best option to do is to beat in the butter whilst taking the pan on and off the heat and this will prevent the sauce from splitting.
Season lightly with a little salt and pepper and, if desired, you can add some freshly snipped herbs.
Ensure that butter is cold as the cold butter meeting the hot liquid is what causes the sauce to thicken.
It is vitally important that you continue whisking at all times when adding the butter to ensure that the sauce does not split.
If you wish when you are reducing the wine/shallots etc you could add some bay leaves or black peppercorns for added flavour.
If you need to store this sauce for a few minutes before serving, store it in a china teacup or in a glass jug set in a saucepan of warm water.
Wholegrain mustard, chopped herbs, capers or diced chorizo can be whisked into the sauce before serving for a varied finish.
People often thin this sauce down with a little fish stock or chicken stock.
Why not cook some prawns and add them to the sauce to serve over some pan fried fish.
For a champagne beurre blanc replace the white wine with some champagne.
Beurre Nantais is when you add in some cream to the sauce also.