Peel and core pineapples, discarding only the peel.
Chop the cores and puree with 1/2 cup of water in a blender.
Strain using a sieve and pressing down on the solids to extract the most liquid.
Discard the solids and set the juice aside.
Chop the remaining pineapple pieces and set aside as well.
Heat 1 tbsp oil in a large pot, then fry the chillies and 2 tsp garlic for a minute or two until fragrant.
Add 1/4 cup fish sauce and cook another 2 minutes to reduce.
Add enough water to the pineapple juice to make up 8 cups, then add to the pot with the lemongrass stalk.
Bring to the boil, then add mushrooms, tomatoes, reserved pineapple pieces and 1/2 tsp pepper.
Return to the boil, then cover and allow to simmer for 10 minutes.
Meanwhile, heat the remaining oil in a frying pan, add rest of garlic, along with a pinch of chilli flakes and the remaining 2 tbsp of fish sauce.
Cook 2 minutes or until reduced, then season chicken with 1/2 tsp pepper and add to the pan, frying briefly until just cooked.
Stir chicken into the soup and simmer for 1 minute.
Divide noodles amongst serving bowls, then ladle in the steaming hot soup.
Sprinkle with chopped herbs and serve.
For more interesting recipe combinations from The Creative Pot click here...