Clean the carrots, onions, and leeks, then cut into small cubes. Heat the rapeseed oil in a pan over medium-high heat and roast the vegetables for 5 minutes, stirring occasionally.
Cut the mushrooms into slices roughly 1 cm thick. Finely chop the garlic. Add both to the pan and roast for 3 minutes, stirring occasionally.
Stir in the sugar, thyme, and tomato paste and cook for 1 minute. Season with salt and pepper. Pour in the wine, reduce heat to low, and simmer for 15 minutes, stirring occasionally.
When the sauce is dark and thick, stir in 2 tablespoons of butter (or to taste). Serve sprinkled with parsley and accompanied by boiled potatoes.