Boil the potatoes until tender, drain the water, add a teaspoon of salt, a pinch of pepper, 4 tablespoons of butter and mash, gradually adding the cream while stirring. Cover and set aside.
In a large skillet over low heat, melt the remaining butter and gradually add the vegetables: onion, then after 10 minutes the garlic, carrots in a minute, and then the remaining vegetables: celery, fennel, cabbage, turnips. At the end put the thyme, the remaining salt and pepper and mix well. Pour in broth and simmer over medium heat, covered lid for about 10 minutes then remove the lid, increase the heat, simmer so until cooked vegetables, evaporated to a liquid.
warm oven in a baking dish Put the vegetables on top Distribute mashed potatoes, if desired, sprinkle with grated cheese and bake until brown 5-10 minutes. Serve the pudding hot with sour cream or simply wrong.