In a small bowl combine catsup, pickled peppers, relish, onion, and poppy seed. Cover and chill up to 3 days.
To grill by direct heat method:
Grill frankfurters on an uncovered grill directly over medium coals for 8 minutes. Turn frankfurters and brush with some of the catsup mixture. Grill for 6 to 8 minutes more or until heated through.
To grill by indirect heat method:
Arrange preheated coals around a drip pan in a grill with a cover. Test for medium heat above pan. Place frankfurters on grill over pan. Cover and grill for 15 to 20 minutes or until heated through, brushing occasionally with some of the catsup mixture.
To serve, toast cut sides of buns on grill. Serve frankfurters in the buns and top each with remaining catsup mixture. Makes 4 servings.