Cut the unpeeled potatoes into slices and cook in salted water. At the same time peel and slice the onions. Pluck the sage and fry with the onions and a teaspoon vegetable stock in a pan until translucent. Add the boiled potatoes and mix everything in a baking dish. Now just add the cream and sprinkle with grated Parmesan.
Bake in the oven at 200 degrees until the cheese is melted and golden brown.