Preheat the oven to 275 degrees F (135 degrees C).
Melt butter in a large skillet over medium-high heat. Add onion; cook and stir until translucent, about 5 minutes.
Stir in ketchup, water, vinegar, brown sugar, Worcestershire sauce, and mustard; simmer about 10 minutes.
Stir in ham; cook until sauce is bubbling and ham is heated through, 5 to 7 minutes.
Meanwhile, heat kaiser rolls in the preheated oven until toasted, 5 to 7 minutes.
Divide ham mixture among toasted rolls; top with pickle relish. Serve hot.
