Thinly peel the kohlrabi with a vegetable peeler. If they still have their green parts on, cut them off and set to one side.
Bring a pot with water to the boil and cook kohlrabi, covered, till al dente, about 30 to 40 minutes.
While the kohlrabi cook remove the crust from the bread and soak the bread in a little water, then squeeze to remove excess liquid. In a bowl mix mince, onion, egg, mustard and bread and season well with salt and pepper.
Remove kohlrabi from the pot with a slotted spoon and reserve the cooking liquid. Cut off the top of each kohlrabi like a lid and hollow it out with an apple corer or a small sharp knife. Make sure not to cut into the outside. Fill the kohlrabi with the meat mixture.
Preheat your oven to 200 C / Gas 6.
Measure out 250ml of the cooking liquid, add the granules and stir till dissolved. Pour into an ovenproof dish. Place the stuffed kohlrabi in the dish and bake in the preheated for 25 minutes.
Remove kohlrabi from the dish and keep warm. Pour liquid into a small saucepan, add cream and wine and season with salt, pepper and nutmeg. Bring to the boil, then whisk in the thickening granules and let simmer till thickened, stirring constantly. Stir in parsley and the minced kohlrabi greens if you have any. Serve the sauce with the kohlrabi.