Heat the oil in a large saucepan, add the grated onion, garlic, ginger, lemon grass, chilli powder and turmeric and cook for a few minutes over a medium heat, stirring, until fragrant.
Add the water, fish sauce, quartered onions and rice flour mixture.
Mix well and bring to the boil, stirring thoroughly to prevent any lumps forming.
Once the soup has thickened, reduce the heat and simmer for 20 minutes.
Cut the fish into chunks then add to the soup. Mix well and continue to cook for a further 10 minutes.
Meanwhile, bring a large pan of water to the boil, add the noodles and cook for about 5 minutes or until tender. Drain well.
Place a portion of noodles in individual soup bowls and top with the soup. Serve hot with the garnishes served in separate dishes and allow diners to add whichever of the garnishes they like to their own bowl of soup.