Melt The Butter In A Large Skillet Over Medium Heat. Saute The Ham, Onions And Pineapple Chunks In The Butter For About 5 Minutes. In A Separate Medium Bowl, Combine The Pineapple Juice, Vinegar, Brown Sugar, Mustard And Cornstarch. Stir This Together Well And Pour Over The Ham Mixture In The Skillet. Stir Well And Allow To Heat Through And Thicken, About 5 Minutes.