Melt the butter in a saucepan over medium heat. Add the sliced onions and sugar and cook for about 30 minutes until caramelized (deep golden).
Add the red wine and herbs, bring to a rapid boil for 2 minutes, reduce the heat and simmer for 10 minutes.
Add the stock, mustard, salt, pepper and Worcestershire sauce, bring to a boil, reduce the heat to a low simmer and simmer, covered, for 20 minutes.
Remove sprigs of herbs. Add the vinegar and simmer for another minute or two.
Melt 50g of butter over a low heat in a suitable size pan. Add 50g plain flour and heat gently stirring continuously to make a roux. Once it looks smooth and glassy turn up the heat and very gradually add the onion mixture, a little at first, stirring constantly.
Allow to simmer gently to cook out the flour, stirring regularly.
Add the cold butter and whisk until dissolved. Add salt and pepper to taste.