Melt the butter in a large rimmed sauté pan on medium-high heat. Heat until butter is completely melted and beginning to bubble. Add the onions and then sprinkle with sugar. Reduce heat to medium and cook onions for 30 minutes, or until caramelized, stirring occasionally to keep the onions from sticking to the pan.
Pour the red wine into the caramelized onions and place the herbs on top. Bring onion mixture to a rapid boil for about 2 minutes and then reduce heat to simmer for 10 minutes, or until liquid has cooked down.
Add the beef stock, mustard, salt and pepper, and Worcestershire sauce, and bring to a boil for 1 minute. Cover the pan, reduce heat to simmer, and cook for 20 minutes. Remove herb sprigs.
In a small bowl, combine the balsamic vinegar and corn starch until smooth to make a slurry (thickener). Pour the slurry into the pan and whisk constantly until gravy has thickened. If gravy becomes too thick, you can add butter or broth , a little at a time, to thin it out. If the gravy is too thin, make another slurry, but this time using water and corn starch and whisk into the mixture.
Serving Suggestions:Onion Gravy is best with Bangers and Mash (also known as Irish Sausages and Mash), however; it is also delicious over roast beef and mashed potatoes, or as a gravy for Salisbury steaks!