bouquet garni (parsley stalks, thyme & bay leaves all fresh if possible, tie with un-waxed kitchen string & wrap with muslin (cheese cloth) you can also add a few black pepper corn if desired, i also add 3-4 whole cloves)s
Place corned beef into a large saucepan with vinegar, brown sugar & bouquet garni. Pour in enough water to cover joint completely.
Bring slowly to the boil. Reduce heat. Simmer gently, covered, for 1 1/2 - 2 hours or until a fork inserted into the meat comes out easily.
Add potatoes, onions & turnips simmer for 20 minutes, add carrots and simmer for another 15 minutes. Add cabbage and simmer until the cabbage is cooked to desired texture. Remove from heat and allow meat and vegetables to cool in liquid.
PREPARE SAUCE: Heat butter & oil in a medium-sized saucepan. Saute onion over medium heat for 3 minutes or until tender. Stir in flour & mustard. Cook stirring for 1 minute
Remove from heat. Gradually blend in combined milk & cream. Return to heat. Cook, stirring, until sauce boils & thickens. Simmer for 3 minutes. Add parsley and seasonings.
Remove meat from liquid and slice & arrange on serving platter & top with sauce, arrange other vegetables around the meat and serve.