Clean the spring onions and cut into rings. Finely chop the herbs. Mix the kefir, spring onions, heavy cream and flour in a saucepan. Heat this mixture, stirring constantly, and cook, continuing to stir, until it thickens. Break the butter into small pieces and stir into the soup with the whisk of a hand mixer.
Mix in the red bell pepper flakes and herbs with a wooden spoon. Season with salt and cumin and serve very hot.
