To make the stock:
Bring 1 litre of water to boil in a large saucepan. Add the prawns heads only and simmer for 15 minutes. When 15 minutes is up, remove the heads - this liquid becomes the stock for the soup.
Meanwhile, heat the oil in a pan and saute the garlic, lemon grass, lime leaves, shallots, galangal and chillies for 3 minutes over medium heat.
Add the chilli paste and stir for additional 1 minute then pour in the lime juice. Let simmer for 5 minutes.
Pour the spicy mixture from the pan into the stock and bring back to the boil. Add the squid, baby corn, mushrooms, tomatoes and prawns.
Turn down the heat, pour in the coconut milk and let simmer for 5 minutes. Add salt to taste and serve hot.