Clean and wash the Brussels sprouts and cut the stem ends crosswise. Wash the duck breast, dab dry and rub the meat side with marjoram and pepper. Brush a pan with oil, heat it up and fry the duck breasts first with the skin side down for 8-10 minutes.
Turn breasts, fry briefly on the meat side and season with salt and pepper. Fry over low to medium heat for 15-20 minutes, turning. In the meantime spread out the frozen Rösti on a baking tray.
Bake in a preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: level 4) for approx. 20 minutes until crispy. Turn once after 12 minutes. Put the Brussels sprouts in little boiling salted water and cook covered for about 20 minutes.
Wrap the finished duck breasts in aluminium foil and let them rest for about 5 minutes. Drain the fat from the pan and deglaze the roast with cognac, duck stock and cream. Add peppercorns. Bring the sauce to the boil, let it boil down for about 3 minutes.
Add the sauce thickener while stirring, bring to the boil again briefly. Season to taste with salt and pepper. Drain the Brussels sprouts, toss briefly with fat and nutmeg. Cut duck breast into slices and arrange on plates with cognac-pepper cream sauce, Brussels sprouts and roasting wedges.
Sprinkle with orange-peel julienne as desired and serve garnished with parsley.