Heat the oil in a large saucepan, add the curry paste and cook for 1 minute, then add the stock, coconut milk, lime leaves and fish sauce. Bring to the boil, reduce to a simmer and add the sweet potato. Cook for 12–15 minutes until soft.
Add the rice noodles and sugarsnap peas and heat through. Ladle into bowls and top with the bean sprouts and herbs.