Cut the kohlrabi into thin slices, as well as the carrots. Finely chop the onion.
Heat the butter in a saucepan and sauté the onion, add the carrots and kohlrabi and sauté briefly. Sprinkle some sugar and pour in the broth. Season with pepper and grated nutmeg. Cover everything and let simmer for 15 minutes.
Just before the end of the cooking time, add the peas. Fold in the creme fraiche. Season to taste with salt, pepper, lemon juice and possibly a pinch of sugar.