For the leg of goat: Place the meat in room temperature for 1 hour before cooking. Preheat the oven to 400°F (200°C). In a small pot over medium heat, combine rape-seed oil, parsley root, rutabaga, carrot, and red onion. Cook gently until the vegetables are almost tender. Add a sprig of thyme, and salt and pepper to taste. Set aside.
Place the deboned leg of goat on your cutting board, spreading the meat so that the opening where the bone has been removed is exposed. Sprinkle with salt. Fill with as much of the root vegetable stuffing as you can fit inside, and close tightly with string.
Place the leg on a rack in the oven and bake for 1 hour and 45 minutes, or until core temperature has reached 150°F (65°C). The exact time depends on the thickness of the meat and the intensity of heat in your oven. Let rest in a warm place for 10 to 15 minutes before carving it.
For the beets: In a medium pot, bring the vegetable stock to a boil. Add the beets and red wine vinegar, and boil until tender. Strain, set aside and keep warm.
For the winter cabbage and Brussels sprouts: In a large pot, bring water to a boil. Add a pinch of salt. Fill your kitchen sink with cold water. Rip the leaves from the winter cabbage. Cut the Brussels sprouts in two, lengthwise. Boil the cabbage for 5 minutes and transfer to cold water to stop the boiling process. Repeat with Brussels sprouts.
In a large sauté pan, heat the rape-seed oil over medium high heat. Add garlic, red onion, cabbage and Brussels sprouts and sauté for 6 to 8 minutes or until the cabbage starts to collapse. Serve thin slices of meat along with the beets, winter cabbage and Brussels sprouts and season with salt and pepper.