Peel and wash the potatoes and cook them in boiling salted water for about 25 minutes. In the meantime peel and finely dice the onion. Heat 1 tablespoon of oil in a pot and fry the onions until transparent. Add lentils, deglaze with 900 ml water, bring to the boil and cook covered for about 25 minutes.
Meanwhile, clean and wash the pumpkin and cut into fine cubes. Wash the flesh, dab dry. Cut the skin side crosswise. Heat 2 tablespoons of oil in a pan. Fry duck breast for 3-4 minutes on the skin side.
Turn the meat and fry for 1-2 minutes, season with salt and pepper. Place the duck breast on a baking tray lined with baking paper. Roast in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 10-12 minutes.
Add the pumpkin to the lentils, cover and cook for another 10-12 minutes. Season to taste with salt and pepper. Mix in the cream, bring to the boil again briefly. Season to taste with lemon juice. In the meantime heat milk and butter.
Drain the potatoes, add the fat and milk mixture and mash with a potato masher. Season with nutmeg. Remove duck breast from the oven, wrap in foil and let rest for about 5 minutes, cut open.
Arrange vegetables and duck breast on plates garnished with oregano. Add mashed potatoes in a bowl.