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Heat the oil in a large saucepan. Fry the curry paste and onions together over a medium heat for 3 mins, stirring. Add the parsnips, lentils and apple pieces. Pour over the stock and bring to a simmer. Reduce the heat slightly and cook for 30 mins, stirring occasionally, until the parsnips are very soft and the lentils mushy.
Remove from the heat and blitz with a stick blender until smooth. (Or leave to cool for a few minutes, then blend in a food processor.) Adjust the seasoning to taste. Heat through gently, then ladle into deep bowls. Serve with natural yogurt and garnish with fresh coriander, if you like. (For freezing instructions see below left.)