Clean and rinse Brussels sprouts. Peel and finely dice the onion.
Heat oil in a saucepan. Sauté onion until translucent.
Add Brussels sprouts and steam briefly. Extinguish with 200 ml water. Add clear soup. Allow to boil up and cook with closed lid
simmer for 10 minutes.
Remove peel from potatoes, rinse, grate coarsely and mix with flour. Fill Brussels sprouts together with clear soup into a gratin dish. Spread potato flakes evenly on top. Season whipped cream with salt, pepper and nutmeg. Pour evenly over the cabbage.
Bake the casserole in a heated oven (electric oven: 200 °C / convection oven: 175 °C / gas: level 3) for 40-45 minutes. Take out and sprinkle with nut flakes.