Sauté the onions, finely chopped, in olive oil (in a saucepan large enough for the soup). Then add the broth. Now add the potatoes (peeled and diced) and simmer covered for about 20 to 25 minutes.
Now add about two thirds of the cress and puree the soup. Squeeze half a lemon and add the juice. Season with salt and cayenne pepper and add the rest of the cress. Serve with croutons and / or cream.