1) Slice the ginger and remove the outer layer of the lemongrass and bash a bit but leave whole. 2) Add 1 can of coconut milk to a pan, place on a medium heat and put in the lemongrass and ginger. 3) Slice the chicken breast into chunks and as the coconut milk comes to a simmer add the chicken to the pan along with the other can of coconut milk. 4) Turn down the heat as you don't want the coconut milk to boil and burn. 5) Slice the chillies diagonally and add to the pan, then add the oyster mushrooms. 6) Keep the coconut milk at a very gentle simmer and only stir in one direction to prevent curdling. 7) Slice the onions and tomatoes into wedges and add to the pan, simmer for 5 minutes or so then turn off the heat. 8) Add the lime juice and coriander and check that the flavour is salty and sour enough as you can always add more salt or lime juice.