Slice the onion and green pepper as thinly as possible (a mandoline slicer is quick and even). Cut the wiener sausage into thin rounds.
Spread a good layer of ketchup right to the edges of the bread.
Scatter the onion and green pepper over the ketchup, then arrange the sausage rounds on top.
Cover with a generous handful of shredded cheese and add a pinch of herb salt or Italian seasoning if you like.
Toast in the toaster oven until the cheese is melted and bubbling and the edges are golden, about 5-7 minutes. Once the bottom of the bread has browned, slide a piece of aluminum foil underneath so the base stops toasting while the top keeps melting.
Serve hot, while the cheese is still molten.