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.
Add the onions and sweat, covered for 5-10 minutes until softening
Add apple, garlic, coriander stalks and ginger, season, then gently cook for 5 mins, stirring every so often.
Add sweet potato, stock, lentils and milk into the pan, 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