Spicy Thai Style Broth with King Prawns and Chicken

I've adapted the famous Tom Yam Kung recipe to become a little more substantial using some bok choy for crunch.  Fingers crossed it works.  I'm told the trick with Thai flavours is to get the balance right between, hot, sour, sweet and salt.  The only way I've ever managed this is by tasting, tasting and tweaking until it's spot on.  You'll soon have a festival of flavour in only a few minutes.

Ingredients
600ml chicken stock
1 stalk of lemon grass, chopped in 4 and bruised
4 kaffir lime leaves, rolled and thinly sliced
2cm ginger, grated
2 small red chillies, thinly sliced
8 small white mushrooms, sliced
1 chicken breast, VERY thinly sliced
8-10 raw king prawns, peeled and deveined
2 tbsp fish sauce
2 tbsp lime juice
1 teasp sugar
1 head of bok choy, white parts sliced, green part left whole
2 spring onions, thinly sliced at an angle
Coriander leaves, as garnish.

Method
Pour the stock into a large saucepan. Add the mushrooms, chilli, lemongrass, ginger and lime leaves.  Bring to the boil and then add the chicken first and cook for one minute, next add the fish sauce, lime juice, sugar, bok choy, half the spring onions and finally the prawns.  Simmer gently until the prawns are pink and cooked.  This should only take a minute or so.  Taste, taste and tweak the flavourings and finish with a flourish of coriander leaves and the remaining spring onions.