Preheat the oven to 350°F. Generously brush the outer sides of the hot dog buns with melted butter, then evenly sprinkle them with poppy seeds. Arrange the buns seam-side down on a baking sheet and bake for approximately 10 minutes, or until they are lightly toasted and the poppy seeds adhere.
Bring a large pot of water to a rolling boil. Carefully add the hot dogs and let them simmer for about 5 minutes, ensuring they are thoroughly heated.
Nestle each hot dog into a prepared bun. Drizzle yellow mustard over the top, then scatter finely chopped onions and vibrant pickle relish for a burst of flavor. Place tomato slices along one side of the hot dog, followed by a crisp pickle spear on the other. Tuck two sport peppers on each side and finish with a light dusting of celery salt for an authentic touch. Serve immediately and enjoy this iconic Chicago favorite.