Peel and halve the onion and cut into thin strips. Roast the onions with the oil in a pan for about 10 minutes over a low heat. At the end, season with salt and pepper to taste. Fry the sausages for about 5-6 minutes.
Take sandwich toast (not toasted) in your hand. Slightly bend in the middle. Onions on it. Then a sausage on it. Add ketchup if you like.