Peel the potatoes and kohlrabi and cut into thin sticks. Spread in a sufficiently large flat gratin dish or on a deep baking sheet, pour in the vegetable stock and white wine. Salt, pepper and bake in an oven preheated to 200 degrees for 30 minutes.
In the meantime, heat the butter in a saucepan and stir in the flour. Top up with the milk and cream, season well with salt, pepper and nutmeg and cook for 10 minutes while stirring.
Add the washed and chopped herbs to the sauce and then spread over the potato and kohlrabi mixture. Spread the mozzarella cut into small cubes on top and bake for another 10 minutes.