Dice the chicken into bite size cubes. Marinade with oyster sauce. Leave in refrigerator for 30 minutes.
Slice the ginger up into really thin strips.
Remove the outer leaves from the lemon grass until you get to the soft bit in the center. Now take a rolling pin or something heavy and batter the lemongrass.
Place the water in a pot and start to heat, while the water is heating, juice your lime. When the water is getting hot, put the lime juice, lemongrass, galangal and fish sauce inches And bring to the boil.
Once it starts to boil add the chicken and coconut milk, stir and bring it back to boil.
Once boiling, reduce the heat and simmer for about 2 minutes or until the chicken is cooked through. Keep stirring whilst doing this to avoid a skin forming.
Serve the Tom Kha Gai when ready and garnish with the coriander and eat with rice.