Wash the potatoes thoroughly. Cook in boiling salted water for about 20 minutes. Afterwards, depending on the size, cut in half if necessary. In the meantime, clean and wash the cauliflower and cut it into small florets.
Pour into 1/2 litre of boiling salted water. Season with nutmeg. Cook for about 10 minutes. Then drain and store the stock. Peel and finely dice onions. Cut bacon into small pieces. Grate cheese.
Melt the fat. Sauté the onions in it. Add flour and let it sweat. Add cauliflower stock and cream, stirring constantly, bring to the boil. Melt cheese in it. Season to taste with salt, pepper and nutmeg.
Fill potatoes and cauliflower into an ovenproof casserole dish. Pour sauce over it. Spread bacon on top. Bake in a preheated oven (electric: 200 °C/ gas: level 3) for about 20 minutes. Serve sprinkled with chervil.