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Put the curry powder into a large saucepan, then toast over a medium heat for 2 mins. Add the olive oil, stirring as the spice sizzles in the pan. Tip in the onions, apple, garlic, coriander stalks and ginger, season, then gently cook for 5 mins, stirring every so often.
Meanwhile, peel, then grate the sweet potatoes. Tip into the pan with the stock, lentils, milk and seasoning, then simmer, covered, for 20 mins. Blend until smooth using a stick blender. Stir in the lime juice, check the seasoning and serve, topped with roughly-chopped coriander leaves.