In a dry, non-stick frying pan, cook the walnut pieces for 2-3 mins until toasted, turning frequently so they don’t burn. Take off the heat and allow to cool. Put the onion, carrots, apples, vinegar and stock in a large saucepan and bring to the boil. Reduce the heat and simmer for 10 mins, stirring occasionally. Once the onion is translucent and the apples start to soften, add the kale and simmer for an additional 2 mins. Carefully transfer to a blender or liquidiser and blend until very smooth. Pour into bowls and serve topped with the toasted walnuts, and a sprinkling of apple crisps, if you like.