For the sauce, cut the onion into small cubes, put butter in a saucepan and melt. Then fry the onion, turn the hotplate to medium temperature. Add sugar to the onions and let caramelize. Deglaze everything with the vinegar. Add the ketchup, mustard, curry and salt and simmer on a low flame for about 60 seconds.
Bake up rolls according to the instructions on the packaging (can also be normal rolls, but should be warm and crispy).
Fry the sausages in oil or butter.
Cut the rolls open, place sausages in the rolls and add the sauce.