Wash and peel the cucumber and cut into small pieces. Peel and chop the onion. Heat oil in a pot. Fry cucumber and onion in it. Deglaze with broth and bring to the boil. Cover and cook for about 10 minutes.
Wash the borage, dab dry, remove the leaves and chop. Put 1 tablespoon of borage aside, add the rest to the cucumber. Puree the vegetables in the pot with a chopping stick and refine with crème fraîche.
Season to taste with salt and pepper. Fill into soup bowls and serve sprinkled with prawns and borage.