Wash the watercress, shake dry and pluck the leaves from the thick stems. Peel and chop the onions and potatoes. Heat butter in a pot. Sauté onions and potato cubes in it, then deglaze with stock and cream.
Simmer the soup for 15-20 minutes (until the potatoes are soft). Set aside a few leaves of watercress for garnishing, add the rest to the soup. Cook the soup again for about 5 minutes, then puree and season with salt, pepper and possibly some lemon juice.
Before serving, add the crabs to the soup and garnish with watercress. Farmhouse bread tastes good with it.