1.Preheat An Oven To 275 Degrees F (135 Degrees C). 2.Melt The Butter In A Large Skillet Over Medium-High Heat; Cook And Stir The Onions In The Butter Until Translucent, About 5 Minutes. Stir The Ketchup, Vinegar, Water, Brown Sugar, Mustard, And Worcestershire Sauce Into The Skillet With The Onions; Simmer About 10 Minutes. Stir In The Ham; Cook Until Heated Through And The Sauce Is Bubbling, 5 To 7 Minutes More. 3.While The Ham Simmers, Heat The Kaiser Rolls In The Preheated Oven Until Toasted, 5 To 7 Minutes. 4.Assemble The Sandwiches By Dividing The Ham Mixture Among The Toasted Kaiser Rolls, Topping The Ham With Pickle Relish. Serve Hot.