Cut the carrots into slices. Quarter the fennel, eighth the thick fennel. Cook the vegetables in a little salted water until they are al dente. Do not throw away the cooking water.
Peel and dice the onion and sauté in butter. Then deglaze with cream and some vegetable cooking water and let it boil down a little. Season to taste with salt, pepper, nutmeg and stock powder.
Peel and cut the jacket potatoes into eighths and place in a greased baking dish with the vegetables. Pour cream sauce over it and sprinkle with butter cheese. Bake the casserole in the oven at 200 ° C for about 15 minutes.