Melt the butter in a non-stick saucepan.
When bubbling, add the finely sliced onion to the pan.
Fry on a medium to high heat for 10 minutes, stirring regularly until they're soft and have a good amount of colour.
Stir in the flour, mustard and honey on the heat.
When the flour has been absorbed and the mixture is thick, sticky and starting to sizzle, slowly add the stock a little at a time.
Stir continuously on the heat, only adding more stock when it comes to the boil and thickens.
When all the stock is used up, season with salt and pepper.
