Melt the butter in a large, non-stick pan.
Add the onions and fry on a medium heat for 10 minutes, stirring regularly until soft and starting to colour.
Stir in the sugar and fry for a further 5 minutes
Stir in the flour and mustard. Heat for a minute until the flour has disappeared.
Slowly add the stock a little at a time. Stir continuously. Bring to the boil each time before adding more.
When all of the stock is used and the gravy is bubbling, take it off the heat and add some fresh Thyme leaves.
Season with salt and pepper and serve
