Sweat onions and ginger (and chillies) in a pot. Beat the lemongrass with the back of a knife and add to the mixture. Sweat over medium heat for a few minutes.
Deglaze with half a litre of orange juice and bring to the boil briefly. Then add the coconut milk, vegetable stock, soy sauce, sweet chili sauce and banana and boil for 30 minutes over medium heat.