Wash the potatoes and cook in boiling water for 20-25 minutes. Drain, rinse and peel. Cut potatoes in half if necessary. Meanwhile clean and wash the cauliflower, cut the florets from the stalk.
Put the cauliflower in 500 ml boiling salted water, cover and cook for 8-10 minutes. Drain and collect the vegetable water. Peel and finely chop the onion. Melt the fat in a saucepan, add the onions and fry until transparent.
Dust with flour, sweat and gradually add the vegetable water and milk, stirring continuously. Bring to the boil, stir in mustard and 50 g cheese. Bring to the boil again and season to taste with salt, pepper, sugar and nutmeg.
Mix potatoes, cauliflower and sauce. Pour into a greased ovenproof dish. Sprinkle with 50 g cheese and bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 30 minutes.
5 minutes before the end of the baking time, spread the chips and peppers on the casserole.