Heat oil in a large saucepan on medium heat. Cook onions with a pinch of salt for 2-3 minutes until starting to soften. Add garlic and kumara and cook for another 2-3 minutes, stirring regularly.
Add stock and bring to simmer. Cook until kumara is soft (about 10 minutes).
While vegetables and stock are simmering, blanch the watercress (this will help to retain its vibrant green colour). To do this, fill a large bowl with a tray or two of ice cubes and top with cold water. Place watercress in a heatproof bowl or pot and pour over a kettle of boiling water. Leave to sit for 1 minute then drain well and immediately place in the icy cold water. When cooled completely, drain the watercress and add to the pot of kumara. Turn off the heat.
Use a blender or stick blender to blend the soup until completely smooth. Season to taste with more salt (if needed) and a little freshly ground black pepper. Scoop out about ¼ cup soup and mix with crème fraîche to loosen.
To serve, ladle soup into bowls and drizzle with crème fraîche mixture. Garnish with some more fresh watercress and edible flowers if you like.
*Check label if eating gluten free.