Peel the onion. Clean and wash the fennel and chard. Cut onion, fennel and chard stems into fine strips, chard leaves into wide strips.
Fry the sausages in hot oil until brown all around. Take them out. Sauté onion, fennel and chard stems in frying fat. Dust with flour and sweat. Deglaze with vinegar and 150 ml water. Bring to the boil and simmer for about 3 minutes.
Add sausages and chard leaves and heat. Season to taste with mustard, salt and pepper. Serve with toasted bread as desired.