Heat the oil in a large pan, fry the onions until translucent, add the kale, cook for about 15 minutes and season with salt and pepper to taste.
Grease a baking dish and alternately layer the sausage slices, potato slices and kale. Mix the cream and mustard together, pour over them, sprinkle with cheese and bake in a preheated oven at 200 degrees for approx. 40 minutes.