Using the back of your knife lightly bruise the lemongrass and galangal/ginger, and cut into pieces.
Bring the lemongrass, ginger, kaffir lime leaves & chillies to a boil with the chicken broth or water (with a stock cube) in a large saucepan.
Let it simmer for about 10 mins. Then strain the liquid and discard all the solids.
Cut the chicken into bite size pieces and add them to the saucepan and bring it to a boil. Simmer for about 8-10 mins
Add the mushrooms and let it cook further for about 7-8 mins till both chicken and mushrooms are almost done.
Then add the coconut milk, sugar and fish sauce.
Let it simmer for 2-3 mins. Check for seasoning, add a tsp of lime juice for a little tanginess.
Mix ginger, garlic and chilli powder in a bowl. Heat a couple of tbsps of olive oil in a pan and pour on top of the spices. Let it stand for a few minutes before using.
Serve the soup in bowls with chilli oil and coriander leaves for garnish.