Heat the oil in a large saucepan over a medium heat.
Add the onion and cook for 5 minutes, stirring often, until softened.
Add the garlic, galangal paste, lemongrass, lime leaves, chillies, mushrooms, and Thai curry paste. Cook for one minute, stirring often.
Add the chicken stock, coconut milk, fish sauce, sugar, lime juice, and shredded chicken.
Bring to the boil and simmer for 5-6 minutes, until the chicken is fully heated through. If you would like to thicken the soup at all, slowly pour in the cornflour/water slurry, whilst stirring.
Taste the soup and season with a little salt and pepper if needed.
Divide between bowls, then top with fresh coriander and drizzle with a little chilli oil. Top with slices of lime before serving.