In a medium pot, add the stock, lemon grass, coriander, lime leaves, ginger, and tomato paste and simmer for 15 minutes.
Add the tomatoes and mushrooms and simmer for another 10 minutes.
Now add the brown sugar, chilli, fish sauce, lime juice, and simmer for 5 minutes.
Finally, add your prawns, which will cook quickly (usually around 5 minutes and then just before the soup is finished, add your coconut cream.
Season to taste with more lime juice if required and garnish with coriander leaves.