In a medium saucepan, heat 1 tbsp. olive oil and add onion, celery, and lemongrass. Simmer until translucent.
Add coconut milk, chicken stock and bring to boil over high heat. Add lime juice, fish sauce (if using), ginger, sugar and chili paste. Reduce heat and simmer until chicken is firm and white, 5 to 10 minutes. Discard lemongrass.
Adjust seasoning with salt & pepper
Ladle into bowl and garnish with raw mushrooms slices and cilantro leaves