Clean the duck of excess skin/fat and season with salt and pepper. Wrap in baking paper, then in aluminum foil and cook in the oven for 2 hours at 180 °C (356 °F) in a pan with a little water.
Clean and chop the carrots, then boil in salted water.
When the carrots are cooked, drain and mash them with the butter in a blender to create a creamy purée.
Clean the red onion and cut it jullienne, then cook in a saucepan with the red wine over low heat.
When the onion has softened, can add the sugar and continue to cook it until it starts to boil with thick bubbles, but be careful not to burn it as the sugar will turn brown and bitter.
Before serving, remove the duck thighs from the foil and brown for 5-10 minutes in a well heated oven until the skin is golden brown and crispy.