Braise the carrots and kohlrabi in 20 g margarine. Deglaze with 1/4 L water and vegetable stock and add the potatoes. Let simmer for about 20 minutes.
Drain the vegetables and collect the water as it is used for the parsley sauce.
For the sauce:
Fill up the vegetable water with the milk up to 400 ml.
Make a roux out of the margarine and flour and deglaze with the water-milk mixture. Let simmer well. In the meantime, chop the parsley and add to the sauce, season with salt and pepper. Finally add the vegetables.
If you like, you can also make a delicious casserole out of it. Sprinkle the cheese over the vegetables and put in the oven until the cheese is golden yellow.