First peel the onions and then cut them into rings.
Melt margarine in a saucepan and sauté the onions until translucent. Dust with flour and stir vigorously. Add the ketchup and stir again, then add water to everything until the desired consistency is achieved.
Season the sauce with salt and pepper.
We also like to eat mashed potatoes with bratwurst and sauerkraut or liver.