Heat the oil in a large heavy-based saucepan over medium-low heat. Add the onion and cook, stirring, for 3-4 minutes or until the onion is soft. Add the garlic, chilli, spices and tomato paste, and stir for a further minute.

Add tomatoes, lentils and stock, and bring to the boil over high heat. Reduce heat to low, partially cover, and simmer for 20 minutes. Set aside to cool slightly.

Puree the soup in a blender, in batches, until smooth (add 1/2 cup (125ml) water if soup is too thick).

Return to the saucepan and season with sea salt and freshly ground black pepper. Place over medium heat and stir until warmed through. Ladle soup among serving bowls and swirl in a spoonful of yoghurt to serve.