Roughly chop the white part of the lemongrass and place in a small food processor with the onion, chillies, garlic, lime leaves and half a tomato.
Process until finely chopped, scraping down the bowl a few times. Alternatively pound in a mortar and pestle until smooth.
Heat the oil in a large saucepan over medium heat. Add the paste and cook, stirring, for 5 minutes until fragrant. Add the remaining tomato and curry powder and cook for 2 minutes until the tomato softens and begins to break down
Add the potato, carrot, and veg stock, bring to the boil, reduce the heat and simmer steadily, covered, for 15 minutes.
Add the green beans, stir to combine and continue to cook, covered, for a further 7–8 minutes until the carrots and green beans are tender.
Divide curry between bowls and top with the extra chilli, lime leaves and roasted peanuts. Serve with steamed rice.