Place the onion in a bowl with the buttermilk. Stir and marinate for 30 minutes. Heat the oil in a deep pan, then place the flour, paprika, garlic salt, pepper and salt into a small bowl.
Once the oil is shimmering hot, place one third of the onions in the oil. You may need to separate the onion slices with your hands before putting in the pan as they can stick together.
Now it’s time to assemble. Slice open a brioche roll and place the cooked hot dog inside. Top with ketchup and mustard, then sprinkle the crispy onions on top.