Cut the kohlrabi and sweet potatoes into 1 cm cubes and briefly blanch them in boiling salted water. Depending on the size of the cubes, the sweet potatoes take about half the time of kohlrabi.
Cut the onion and optionally the ham into small pieces and sauté in a little oil until translucent. Deglaze with the cream and season with cardamom, curry, cumin, coriander, cayenne pepper and salt and pepper.
Place the diced vegetables in a medium-sized baking dish and pour the sauce over them. Sprinkle the Emmentaler cheese on top and bake in the preheated oven for about 15-20 minutes until the cheese is slightly brown.
Rice or flatbread goes well with this, but the gratin can also be served very well as a main course.
If you like a little less sauce, you can reduce the amount of cream.