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Peel and dice the potatoes, kohlrabi, and onion.
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Heat the oil in a pot over medium heat.
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Add the onions and sauté until transparent.
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Add the potatoes and kohlrabi and sauté for a few minutes.
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Stir in the mustard and cook for 1 minute.
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Add the broth and sour cream.
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Bring the soup to a boil.
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Cover and simmer over medium heat for about 20 minutes.
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Season the soup with salt and pepper to taste.
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Puree the soup with a hand blender or food processor.
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Ladle into bowls and serve with a dollop of mustard on top.