Dice the onion. Trim the green from the kohlrabi, wash and cut into small pieces. Peel the kohlrabi bulb and roughly divide.
Heat the butter in a pan and lightly sauté the onion. Add the kohlrabi and braise with the onions. Sprinkle with sugar, pour on the broth and leave the soup to simmer for about 15 minutes.
Then remove the soup from the heat, purée the kohlrabi, pour in the cream, quickly bring to the boil and season with salt, pepper and lemon juice to taste. Sprinkle with kohlrabi green and serve.