Heat olive oil in a large saucepan over medium heat. Add the spices and cook for 1 minute until fragrant. Add the onion and salt and cook, stirring for 2-3 minutes until tender. Add the garlic and ginger and cook for a further 30 seconds or until fragrant.
Add sweet potato and capsicum and cook for 1 minute. Add the lentils and stock. Bring to the boil, then reduce heat to low and cook for 20 minutes or until sweet potato is tender and the soup thickens.
Season to taste and serve topped with yoghurt and mint.