In a large saucepan, fry the onion and red pepper until the onion is soft.
Add the garlic and coriander seeds and fry for a further 2 minutes.
Stir through the turmeric and chilli powder and heat for 1 minute.
Add the Thai red curry paste and lentils and stir thoroughly.
Add the coconut milk, stock, soy sauce, lemongrass, kaffir lime leaves and sugar. Bring to the boil and then simmer for 20 minutes. You might need to add a bit more stock if it gets too thick.
Remove the stick of lemon grass and kaffir lime leaves before transferring the soup to a blender and then blending until smooth.
Return to the pan, add the lime juice and continue to simmer for 2 minutes. Taste the soup and add more sugar, lime juice and/or soy sauce if needed.
Add any of the optional toppings and serve with crusty bread.