Wash the potatoes and cook in plenty of boiling water for about 20 minutes. Clean and wash the cauliflower and divide it into florets. Cook in salted water for about 10 minutes and remove. Mix 3/8 litres of cauliflower water and granulated stock and let it cool down.
Drain the potatoes, rinse with cold water, peel and quarter them. Cut the sausage into slices. Heat the fat in a pot and sauté the flour in it. Deglaze with cauliflower water and 300 ml milk and simmer for 5 minutes at low heat.
Stir in the cheese. Whisk remaining milk and egg yolk and stir into the sauce. Season with pepper and nutmeg. Layer potatoes, cauliflower and sausage in an ovenproof dish. Pour over the cheese béchamel and bake in a preheated oven (electric: 200 °C/ gas: level 3) for approx. 45 minutes.
Chop the parsley finely, except for something to garnish, and sprinkle over the finished casserole. Serve garnished with the remaining parsley.