Make the watercress puree by melting the butter in a small frying pan over a medium heat. Sweat the onion and garlic for 6-8 minutes stirring occasionally, until softened and translucent in appearance. Add the crème fraîche, heat until thinned then reduce by half. Remove from the heat and set to one side.
Blanch the 200g of watercress in a large saucepan of salted water for 10 seconds, then drain and refresh in a large bowl of iced water. Drain from the iced water and place in a food processor along with the crème fraîche mixture. Blitz until smooth, adjusting the seasoning if necessary with lemon juice, salt and pepper.
Spoon onto the centre of serving plates and top with the crumbled feta. Sit shavings of Parmesan on top and a few watercress leaves as well. Season lightly with black pepper and serve immediately.