Spray a large saucepan with low-calorie cooking spray and set on a medium heat.
Add the onion, garlic and ginger and fry for 4 minutes until just softening. Add the chicken breast, carrot and potato, and pour over 400ml of the coconut milk drink and water.
Bring the pan to the boil, then lower the heat to a simmer. Add the lemongrass paste, fish sauce, red chilli and chicken stock pot.
Season with salt and pepper and add the ground cumin. Simmer the soup for 20 minutes, then add the green beans for a further 5 minutes.
While the soup is simmering, cook the noodles according to the packet instructions, drain and leave to one side.
After 25 minutes remove the chicken breast and shred with 2 forks. Add the basil and coriander to a food processor with the extra 20ml of coconut milk drink. Blitz until smooth, then add to the pan and stir.
Add the chicken back to the pan with the noodles stir and allow to simmer for a further 2 minutes.
Serve sprinkled with the spring onion, a drizzle of sriracha sauce and a lime wedge.