Heat coconut oil in a saucepan and sweat the chopped onion in it. Deglaze with the vegetable stock and add all the other ingredients in the soup - including the lemon quarter as a whole. Simmer for about 20 minutes, until the lentils and sweet potatoes are soft.
Puree the whole thing in a blender until creamy. Season to taste with salt and freshly ground black pepper.
Pour the soup into bowls and serve garnished with spring onions, coriander and roasted sesame seeds.