Tie the thyme, rosemary and sage into a neat bundle using kitchen string (bouquet garni). Melt the butter in a large saucepan over medium heat. Add the onion and sugar, then cook, stirring often, for 30 minutes or until caramelised.
Once your onions are caramelised, increase the heat to medium-high. Add the wine and bouquet garni, then bring to a boil. Reduce the heat to medium. Simmer for 5 minutes to reduce the wine. Add the beef stock, Worcestershire sauce and mustard, and season to taste. Reduce the heat to low. Simmer for 15-20 minutes or until it coats the back of a spoon. Remove and discard the bouquet garni.
Whisking constantly, add the balsamic vinegar and the cornflour slurry to the pan. Simmer, whisking constantly, for another minute until thickened. Add the cold butter and whisk to incorporate into the sauce. Season to taste, then add more mustard, if desired