Put the onion, garlic, carrot and ginger into a saucepan, drizzle over the oil, and cook for a few mins to soften. Add the turmeric, cardamom and cumin. Stir well to combine, and cook gently for a few mins more, until the spices are aromatic.
Add the lentils to the pan. Pour over the coconut milk, fill the can with water and add this too. Bring to the boil, then reduce to a simmer for 15 mins until the lentils are soft and swollen but retain a little bite. Using a hand blender, pulse the soup until some is blended completely, leaving some larger vegetable chunks.
Zest the lemon straight into the pan, then squeeze the juice over the soup and stir through with a pinch of salt, chili and finely chopped herbs. Ladle into bowls and sprinkle over a little more lemon zest, if you like.