Finely dice the onions and garlic and sauté in the butter. Deglaze with the dry white wine and add the vegetable broth. Bring to the boil and simmer for 15 minutes. Stir in cornstarch and bring to the boil. Add the leaves of one bunch of garden cress to the soup. Stir in the Créme fraîche and finely purée the soup. Add the seasoning and lemon juice. Garnish with an additional bunch of garden cress.