Melt the margarine in a sauce pan.
Add finely chopped onion and cook gently without color.
Add the vinegar and reduce completely.
Add the demi-glace and simmer for 5-10 min.
Remove from the heat and add the mustard diluted with a bit of water, add sugar . Do not boil.
Skim and correct the seasoning.
Serve with fried pork chops.