Rinse Corn Beef and trim off any access fat.
Place Corn Beef in Dutch oven cover with water and add the spice packet. Bring to boil, cover and reduce heat for about 3 hours or until tender.
Remove Corn Beef and place on a rack, coat the Corn Beef with the mustard sauce. Place in a preheated 350 degree oven for about 30 minutes. Baste with mustard sauce every 10 minutes.
Place brown sugar, water and mustard in a small sauce pan. Heat until all ingredients are well blended and sugar is dissolved.