Let's hear it for an easy beurre blanc! This one shakes things up, quite literally

This is a classic French sauce I spent ages making when I was a trainee chef. Making it used to be a pain in the arse, but I've come up with the solution...

First of all, fill a vacuum flask with boiling water from the kettle. Put the shallot or onion, wine and vinegar in a small saucepan and boil down by three quarters. While it's reducing, cut the butter up into chunks – some small, some a little larger. When the liquid's reduced, empty the flask and pour the wine and vinegar mix into it (through a sieve if you don't want the onion pieces in the finished sauce).

Drop all the butter in on top of it and screw the lid on. Shake the flask vigorously for about a minute – all the butter will melt and merge with the wine and vinegar to make a lovely buttery sauce.

Add a little salt if it needs it and keep the lid screwed on until you're ready to serve. Give it a little shake before your pour.