Wash the potatoes and cook covered for about 20 minutes. Wash duck breast, dab dry and cut the skin into a diamond shape. Season fillets with salt and pepper. Fry in a pan in portions without fat, first on the skin side, then all around for about 4 minutes.
Take out and place them side by side on a baking tray. Roast in the preheated oven (electric cooker: 100°C/circulating air: 75 °C/gas: see manufacturer) for 1-1 1⁄4 hours.
Quench the potatoes, peel them and let them cool down. Clean and wash the sprouts and broccoli. Cut broccoli into florets. Peel and finely chop the onion.
Bring 200 g cream and milk to the boil. Season with salt and pepper. Cut the potatoes into slices and spread them in a large flat casserole dish. Remove duck breast fillets from the oven and cover the tray with foil.
Increase temperature to 225 °C (circulating air: 200 °C/gas: level 4).
In the meantime, cover the Brussels sprouts and cook them in boiling salted water for about 15 minutes. Pour hot cream-milk mixture over the potatoes. Mix Gouda and breadcrumbs and sprinkle over the potatoes. Bake gratin in the preheated oven for about 8 minutes until golden brown.
Cook broccoli in boiling salted water for about 6 minutes. Sauté onion in 1 tbsp. hot butter until translucent. Stir in 200 g cream, bring to the boil and simmer for about 4 minutes. Season with salt, pepper and nutmeg. Drain sprouts and mix with cream.
Roast the nuts in 1 tablespoon butter. Drain broccoli and mix with the nut leaves. Season with pepper. Remove the gratin. Fry fillets without foil at highest oven heat for about 5 minutes until crispy. Then cut into slices and arrange everything.