Peel and dice the onion. Peel, wash and dice the potatoes. Boil up onion, potatoes and stock and simmer for about 15 minutes. Cut bacon into cubes and fry in a pan without fat for about 4 minutes while turning until crispy.
Clean and wash the radishes and cut them into small pieces, except for 2 pieces. Cut the cress from the bed. Add the radishes and cress, except for a little bit for sprinkling, to the soup and cook for about 1 minute. Puree the soup finely with a hand blender.
Season to taste with salt and pepper. Add crème fraîche and mix gently. Cut remaining radishes into fine strips. Arrange the soup and sprinkle with bacon, cress and radishes.