Melt the butter gently in a large pan over a low heat. Add the onion, season, and allow the onion to sweat for about 5 minutes until it begins to soften. Add the potatoes and cover with the stock. Cook for 15 minutes until the potatoes are tender.
Add the watercress and wild garlic, cooking for about 3 minutes until wilted.
While the soup is cooking gently whip the cream, season with salt and pepper and stir in the chives. Leave in the fridge until you are ready to serve.
Remove the soup from the heat and, using a hand held stick blender, blitz until smooth. SErve immediately in bowls, drizzled with olive oil and with a spoonful or two of whipped chive cream on top.