Prepare Grill For High Heat. With A Fork, Poke Sausages Several Times. Place In A Pot, Cover With Water, And Simmer Over Medium-High Heat Until Cooked Through, About 7 Minutes. Remove From Heat, And Set Aside. Warm Butter In A Skillet Over Medium Heat. Stir In Onions, And Cook Until Soft And Translucent. Stir In Apples, Vinegar, And Brown Sugar; Cook, Stirring Gently, Until Caramelized, About 10 To 15 Minutes. Season With Salt And Pepper. Meanwhile, Place Sausages On Grill (Or Under Broiler), And Cook Until Well Browned. Serve On A Mound Of Caramelized Apples And Onions.