Remove the tough outer stems from the lemon grass, then cut into 2.5 cm (1 inch) pieces.
Place the coconut milk, stock, garlic, ginger, chilli, lemon grass, sugar and lime leaves in a large saucepan and bring to the boil. Add the sweet potatoes, cover and simmer for 10 minutes.
Add the red pepper to the pan and cook for a further 5 minutes.
Stir in the spinach and lime juice, re-cover and cook for 2–3 minutes until the spinach has wilted, then stir in the coriander or Thai basil. Serve immediately with steamed jasmine rice.