Clean and peel the kohlrabi. Wash the delicate leaves, shake dry and set aside. Put the kohlrabi into a pan of boiling salted water and cook over a medium heat for 20 minutes.
Meanwhile, wash and slice the spring onions.
Remove the peas from the pods, rinse under cold water and drain.
Wash the mint, shake dry and finely chop the leaves. Finely chop the kohlrabi leaves.
Drain the cooked kohlrabi and allow to cool slightly. Reserve the cooking water.
Melt the butter in a pan and stir in the flour. Cook, stirring, until pale brown in colour.
Add 250ml of the kohlrabi water and the milk to the pan, and bring to the boil, stirring. Simmer over low heat for 15 minutes, stirring.
Cut a lid from each kohlrabi and carefully scoop out the inside. Cut the lids and the hollowed-out kohlrabi flesh into cubes.
Heat the oil in a large pan. Fry the peas and spring onions over a medium heat for 5 minutes. Add the kohlrabi cubes, cream cheese and mint, and season with salt and pepper.
Juice the lemon. Season the sauce with salt, pepper, freshly grated nutmeg and lemon juice. Add the chopped kohnrabi greens and remove from the heat.
Separate the egg. Whisk the egg yolk and whisk into the sauce.
Pour the pea and cheese mixture into the kohlrabi shells and serve with the sauce.