Pop-overs: All ingredients should be at room temperature. Preheat oven (electric cooker: 225 °C/circulating air: 200 °C/gas: see manufacturer). Melt butter in a small pot, let it cool down a little. Smear 8 depressions of a muffin tray with 1 teaspoon of oil each and place in the hot oven.
Mix the liquid butter, eggs, milk and 1⁄2 TL salt briefly with a whisk. Add the flour at once and mix until the dough just has no more lumps. Do not stir for too long! Pour the dough into a measuring cup.
Remove the muffin tray from the oven. Pour about half the dough into the prepared troughs. Quickly spread 1 teaspoon of goat cheese on each and pour the rest of the dough on top. Bake immediately in the hot oven for about 15 minutes.
Then reduce the temperature (electric oven: 175 °C/circulating air: 150 °C/gas: see manufacturer) and continue baking for 10-12 minutes. Do not open the oven door during the entire baking time!
Peel and chop the onion. Cut off the kohlrabi leaves and set aside. Peel, wash and chop the kohlrabi and potatoes. Heat the oil in a large pot. Fry the onion until transparent.
Steam the kohlrabi briefly. Deglaze with 1 l water and bring to the boil. Stir in stock. Simmer covered for 15-20 minutes.
Wash the kohlrabi leaves and dab dry. Add half to the soup and puree the soup finely with a hand blender. Season to taste with salt, pepper and nutmeg. Cut the remaining kohlrabi leaves into fine strips.
Serve the soup. Sprinkle with kohlrabi leaves. Crumble goat's cheese on top. Serve pop-overs with the soup.