3-4 cans coconut milk (make sure it's the unsweetened kind)    
      3 tbsp chopped scallions    
      1 to 3 tsp lemon grass    
      cilantro (preferably fresh. I sometimes leave this out.  Niels says    
            that's defeating the whole point, but I think it still comes out   
            great)    
      tofu, cubed into smallish pieces.    
      chicken, also cubed to bite size.    
      mushrooms    
      1 carrot, grated    
      juice from loads of limes (8? i can never put in enough)    
      serrano chillies (or any other hot chili pepper, again preferably   
            fresh, but powdered will do)    
      1 tsp galanga powder    

Heat the coconut milk in a pot.  Add everything else.  As the lemon grass is   
inedible, put it in a tea ball and immerse the ball in the soup so you can   
retrieve it later.  Cook until the chicken is done and the soup is hot (30   
minutes?).  Taste to see if it needs more limes (it always does) or more hot   
peppers (it's better to start mild and build up to the desired level of   
spiciness).