For the meatloaf: Preheat the oven to 350 degrees F. Grease a 9-by-5-inch loaf pan with cooking spray.
Set a medium skillet over medium heat and add the olive oil. Add the onions and saute until starting to brown, about 10 minutes. Allow to cool slightly.
Meanwhile, combine the milk, poppy seeds and hot dog buns in a large bowl and toss gently to combine; allow the milk to soak into the buns. Then, add the relish, Worcestershire sauce, kosher salt, celery salt, peppers, eggs, 2 tablespoons mustard and a few turns of black pepper to the bowl with the hot dog buns and stir to combine. Add the cooled onions. Add the ground beef and mix with your hands to combine.
Transfer half of the mixture to the prepared loaf pan, place the hot dogs on top on a diagonal so they run down the center and then cover them up with the remaining half of the beef mixture, smoothing it out on top.
In a small bowl, combine the brown sugar and remaining 4 tablespoons mustard and brush it over the top of the meatloaf. Bake until the internal temperature is 155 degrees F, about 50 minutes. Let sit for 10 minutes.
For the topping: Top with the tomato wedges, pickle slices, a sprinkle of poppy seeds, a sprinkle of celery salt and sport peppers before serving.