Get two pans on medium heat.
Add a chunk of butter to one, and your onion, cooking until caramelized.
In another pan fry your hot dog until cooked, then slice into three long pieces.
Butter the outside of each slice of bread.
Spread ketchup on bottom slice, top with relish.
Sprinkle on havarti cheese, layer hot dog slices, caramelized onions and finish with cheddar cheese.
Spread mustard on inside of top slice of bread.
Form sandwich, fry in a covered pan until golden brown on each side.