Heat 1 tbsp oil in a large saucepan. Add the onion and fry for 8 mins until soft and translucent. Stir in half the garlic, the chilli, cumin and curry powder and cook for 1 min more. Add the lentils and stock, then cover and simmer for 30 mins over a medium-low heat.
Put the coriander, remaining oil and garlic, the cashews and lemon zest in a food processor and blitz with 1-2 tbsp water until semi-smooth. Spoon the chutney into a bowl and set aside.
Stir the spinach into the soup and cook for 5 mins, or until wilted. Tip half the soup into a blender and blitz until smooth, then return this to the pan. Stir through lemon juice to taste.
Ladle the soup into bowls and top with generous dollops of the cashew chutney.