Food Around the World - Hot and Sour Prawn Soup

This dish takes me back to an evening in Hong Kong. Noodle bars are very popular with young people as the are good value for money. I could not read the menu, but thanks to some students happy to practice their English, the menu was translated and a recommendation made for this fabulous meal.
It is quick and easy to make and surprisingly a meal in one rather than just a soup!
Serves 4.

1 litre chicken or vegetable stock
2 red chillies seeded and finely chopped
5cm piece of fresh ginger peeled and sliced
2 lime leaves
2T fish sauce
2 lemon grass stalks
juice of 2 lime, plus extra to serve
1T palm sugar
125g shiitake or oyster mushrooms, sliced
250g large prawns, cleaned and peeled
2 pak choi
200g flat rice noodles cooked
toasted sesame seeds to garnish

Place the stock, chilli, ginger, lime leaves, fish sauce, lemongrass, lime juice and sugar in a large saucepan. Bring to the boil, then cover and simmer for 5 minutes. Strain into a dish and clean the pan. Return the stock and add the mushrooms and prawns, and cook for 3 minutes until the prawns turn pink. Add the pak choi for a minutes until it has wilted.
Cook the noodles and place in 4 bowls. Pour the soup over the noodles and garnish with sesame seeds and serve with lime wedges.