Clean and wash the watercress and put something aside for garnishing. Peel and finely chop the onion. Melt the fat in a pot. Sauté onion and watercress in it. Dust with flour.
Add the stock and milk while stirring. Bring everything to the boil and simmer for about 2 minutes. Puree the soup with a hand blender and season to taste with salt and pepper. Roast the pine nuts in a pan without fat until golden brown, take them out.
Leave ham in the pan until crispy. Arrange watercress soup in bowls. Garnish with some crème fraîche, ham, pine nuts and watercress.