Place a medium saucepan over medium to high heat and add the butter.
60 g unsalted butter
When the butter has melted, add the sliced onions and sugar and cook for at least 30 minutes or until the onions have turned a lovely golden brown and caramelised.
2 large white onions, 2 tsp sugar
Add the red wine and herbs and bring to a rapid boil for at least two minutes and then reduce the heat until it starts to simmer. Leave to simmer for 10 minutes stirring occasionally.
1 sprig thyme, 1 sprig sage, 120 ml red wine
Add to the mustard, salt, pepper and Worcestershire sauce and the beef stock and bring to the boil again and then reduce the heat to a gentle simmer.
480 ml beef stock, 1 tsp Worcestershire sauce, 1 tsp salt, 1/2 tsp ground black pepper, ½ tsp English mustard
Put a lid on the saucepan and leave to simmer for 20 minutes.
After 20 minutes, remove the sprigs of thyme and sage and discard.
Add the corn starch and water mixture to the gravy and also the balsamic vinegar and stir thoroughly to combine. Simmer for another couple of minutes, whisking/stirring continuously until the gravy has sickened to your liking.
1 tbsp cornstarch, 1 tbsp balsamic vinegar
Add the cold butter to the gravy and whisk until melted. This will give the gravy a lovely colour and shine.
28 g unsalted butter
Taste the gravy and adjust the seasoning including mustard if required. Serve over sausages and mash potatoes for one of my personal favourites.
Notes
Note 1: In a small saucer add the corn starch and 1 tablespoon of water and stir to dissolve the corn starch into a slurry. This helps to thicken the gravy.