Peel and dice the kohlrabi and potatoes, peel and chop the onion, clean and chop the mushrooms.
Pre-cook the kohlrabi and potatoes with not too much water and the instant stock for 8-10 minutes, using a sufficiently large saucepan so that there is space for the minced mushroom mixture later.
Sweat the onions in a little oil until translucent, add the minced meat, season lightly with salt, pepper and garlic and fry vigorously. Then add the mushrooms and continue frying until everything is crumbly.
Drain the potatoes / kohlrabi (leave them in the pot), collecting the stock. Add the mince / mushroom mixture and mix carefully with the potatoes and kohlrabi. Pour the mixture into a greased baking dish.
First mix the sour cream with approx. 200 ml stock, then stir in the eggs. Season well with salt, pepper and nutmeg. Pour the topping over the casserole and sprinkle with cheese. Bake in a preheated oven (200 ° O / U, 180 ° fan oven) for 25-30 minutes.