Cover the potatoes and boil them in water for about 20 minutes. Clean the cauliflower, cut into florets, wash and cook covered in 1/2 l boiling salted water for about 8 minutes.
Meanwhile grate the cheese finely. Peel and chop the onion. Drain the cauliflower and collect the vegetable water.
Heat 2 tablespoons of butter in a saucepan. Fry the onion until transparent. Dust with flour and briefly sauté lightly. Stir in milk and vegetable water, bring to the boil and simmer for about 5 minutes. Stir in half of the cheese and melt in it.
Season to taste with mustard, salt, pepper and nutmeg.
Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Quench, peel and quarter potatoes.
Grease a casserole dish. Put potatoes and cauliflower in the dish. Pour the sauce over them. Sprinkle with the remaining cheese and bake in a hot oven for about 30 minutes. Distribute the chips and peppers on the casserole dish about 5 minutes before the end of the baking time.