Melt the butter in a saucepan over medium high 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, whisk in the cornstarch and simmer another minute or two until thickened, whisking continuously. Add the cold butter and whisk until dissolved. Add salt and pepper to taste and more mustard if desired.