Melt 25 g of the butter in the saucepan and add the onion, and when you've stirred it so it's nice and buttery, let it cook on a low heat for about 20 minutes.
It's important not to let it colour, so give it a few stirs from time to time.
Now, using a wooden spoon, stir in the sauce flour and the powdered mustard till smooth, then add the milk a little at a time, still stirring, increase the heat to medium, then gradually add the stock, keep stirring after each addition until its all in.
Now turn the heat down low and let it barely simmer for 5 minutes.
Finally stir in the grain mustard, the rest of the butter and some seasoning, then pour into a warmed serving jug.
This is excellent served with boiled gammon or a bacon joint