With a sharp knife make cuts in the duck breast skin, without touching the meat. Leave at room temperature for at least 30 minutes.
Cut the cleaned and peeled pumpkin into pieces and place it in a baking sheet. Add the cleaned Brussels sprouts, drizzle with olive oil, season with salt and pepper and finely chopped rosemary and bake for 30 minutes in a preheated oven at 220°С.
For the sauce, heat olive oil and 1 tbsp of the butter in a saucepan. Add the finely chopped shallot and cook for 3-4 minutes until soft.
Add the sliced plums and cook for another 2-3 minutes. Pour in the stock and wine and bring to the boil. Cook for 10-15 minutes over medium heat until the sauce is reduced by half.
Add the honey and the starch dissolved in 1 tbsp. water, season with salt, season with black pepper and cook for another 2-3 minutes. Add the remaining butter and remove the pan from the heat. Puree and strain the sauce and keep warm until ready to serve.
Remove the pumpkin and cabbage from the oven, separate the cabbage into a bowl and puree the pumpkin with the butter until smooth.
Pat dry the duck breasts with a paper towel. Salt and season both sides and place in a cold pan, skin side down. Cook for 5-6 minutes over medium heat to render the fat and crisp the skin.
Turn the meat over and place the pan in a preheated oven at 200°C. Cook for 6 minutes for medium rare or 8-9 minutes for medium well.
Remove the pan from the oven and transfer the meat to a cutting board. Let it rest for 8-10 minutes before slicing to allow the juices to redistribute. Cut the meat in half lengthwise or into 1 cm thick slices.
Place the pumpkin puree on a serving plate, add the cut duck breasts and the halved Brussels sprouts, and pour over some of the sauce.