Heat the oil in a large saucepan over a medium-high heat. Add the tomatoes and cook for 3-4 mins until soft and saucy.
Add the stock, lemongrass, shallots, coriander stalks, red pepper, garlic, star anise and lime leaves, if using. Bring to a simmer and bubble over a medium heat for 15 mins to infuse.
Add the mushrooms, pak choi and prawns and cook for 3 mins until the prawns turn pink and are cooked through. Stir through the fish sauce.
Discard the lemongrass. Serve the soup in bowls, garnished with the spring onions, coriander leaves, chilli and a squeeze of fresh lime.
Tip: To make this soup more filling, add cooked rice noodles to the broth to serve.