In a large saucepan on a medium heat, melt the butter and gently cook the chopped onion and garlic for 2-3 minutes, or until soft and translucent but not coloured. This gentle sweetening of the onion and garlic will transform the natural starches present into sugar giving that lovely sweetened onion flavour as opposed to the harsh raw onion flavour.
Increase the heat to high, add the watercress and a pinch of salt, cover with a lid and cook for 30 seconds. Add the spinach and cook for 1-2 minutes, or until wilted. The spinach will round up the strong peppery qualities of the watercress.
Pour in the boiling water and simmer for 2-3 minutes, then remove from the heat and add the ice. This quick cooking of the vegetables and cooling with the ice will not only retain the colour and the flavour, but also maximise the vitamins and minerals.
Pour half the soup into a food processor, and blend until smooth. Transfer the soup into a clean pan, then repeat with the remaining soup.
When ready to serve, reheat the soup and season, to taste, with salt and freshly ground black pepper if required. Serve the soup in a large tureen with the crème fraîche swirled through.