Heat the oil and butter in a pan and gently fry the onion and garlic until softened, about 3-4 minutes.
Pour in the vegetable stock and bring to a gentle simmer. Add the cream and the watercress, stir well and bring to a simmer. Allow to cool slightly, then blend with a hand-blender until smooth.
To serve, place the chopped bacon into a pan and fry until crisp.
Pour the soup into a shallow serving bowl. Garnish with a dollop of yoghurt and sprinkle over the bacon.