Heat the oil in a saucepan and add the onion. Stir over a low heat for 5-6 minutes until soft. (Cook on low for longer if you have time).
Add the flour and stir for 1 minute until the onions are coated.
Turn the heat up a little to medium and slowly add the beef stock, stirring all the time.
Add the mustard, thyme, Worcestershire sauce and salt and pepper and simmer for 10 minutes until thickened.
If you have a hand blender, blend until smooth.
Did you make this recipe? Click here to let us know how it went and give it a ⭐️ rating! Your feedback helps us to make more recipes you really want.