Peel the kohlrabi and put tender leaves aside. Pre-cook the tubers in sufficient salted water for 20-25 minutes, depending on size and quality. Cool slightly. Then scoop out the tubers except for one edge with a teaspoon or possibly a Parisian spoon. Cut the kohlrabi flesh into small cubes.
Clean the mushrooms dry and slice them. Peel and finely chop the shallots. In a wide frying pan, sauté in the butter until translucent. Add the mushrooms and sauté for 3-4 minutes, turning. Add the diced kohlrabi and sauté briefly. Add the soup and let it boil. Stir some hot liquid with the sour cream and add it. Add the kohlrabi leaves and the parsley. Season everything with salt and freshly ground pepper.
Fill the kohlrabi with part of the mushroom vegetables and place in a well-buttered baking dish. Spread the rest of the vegetables and sauce evenly in the dish. Cover the dish with aluminum foil or a lid.
Bake the kohlrabi in the oven heated to 200 °C on the second shelf from the bottom for 40 minutes.