Pour 8 cups of water in a deep bottomed pan and keep it on the stove to boil. Add lemongrass, Onions, garlic, galangal and kaffir lime leaves to the pot and let boil for 10 minutes. till they release their flavor.

Mean while place the tomatoes in boiling water for 5 minutes so that their skin can come off. Once the skin comes off, dice the tomatoes into big cubes and keep aside.

Once the stock is boiled, strain it a bowl and throw the lemongrass. Lemon grass and lime leaves are just added for flavorings and they are not eaten with the soup. you can discard them at this stage or while having the soup.

In the same pan, heat oil, then add the chili paste, add Mushrooms and give everything a quick stir and then add the stock back to the pan. Let the soup boil for 5 minutes and then add Tofu, soy sauce,  salt , sugar and tomatoes. Let this boil for a minute more and then add green chilies. 

Turn off the flame and then add lime juice. Garnish with fresh Cilantro leaves and serve it hot.