Peel and finely chop the onion. Heat the oil in a pot and sauté the onion in it. Add curry, sweat briefly while stirring and deglaze with approx. 75 ml water. Add ketchup, heat briefly and set aside.
Just before serving, heat water in a pot and warm the sausages in it. Warm up the curry sauce as well. Arrange 2 or 3 sausages in small bowls with some curry ketchup and sprinkle with plenty of curry. Serve with small table rolls or baguette