Clean, peel and chop the kohlrabi and the onion. Put the oil in a saucepan and fry the onion and kohlrabi in it. Then add the peas and fry them briefly, season with salt and sugar. Deglaze with the water, bring to the boil and simmer for 20 minutes with the lid closed.
Stir in the crème fraîche and puree the soup. Season with salt and pepper, if necessary with a little broth.