Cook the potatoes in salted water for 5 mins. Add the asparagus and kohlrabi, cook for approx. 5 mins. Drain the vegetables, place in the prepared dish along with the onions, season.
Heat the butter in a pan. Add the flour and cook, stirring with a whisk; the flour must not take on any colour. Remove the pan from the heat. Pour in the milk, bring to the boil while stirring, reduce the heat, season, simmer for approx. 10 mins., stirring occasionally until the sauce is thick and creamy. Stir in half of the cheese and herbs. Pour the sauce over the vegetables, scatter the rest of the cheese on top.
Approx. 35 mins. in the centre of an oven preheated to 200°C. Garnish with the remainder of the herbs.