Heat oil in skillet. Add onion; sauté over medium-low heat until tender, 8 minutes. Add sauerkraut and caraway seeds. Cover; cook over medium heat, stirring occasionally, until sauerkraut is tender and lightly browned, about 20 minutes.
Meanwhile, combine ketchup, mayonnaise and relish in small bowl.
Split rolls; do not cut through. Place pastrami down center, dividing evenly. Top with sauerkraut, sauce, cheese. Wrap in foil. Leave top open.
Broil 8 inches from source of heat until heated through and cheese is melted, about 3 minutes. Serve. Makes 6 servings.