Heat a Dutch oven over medium heat. Add oil to pan; swirl to coat. Add mushrooms, bell pepper, ginger, garlic and lemongrass; cook 3 minutes, stirring occasionally.
Add chile paste; cook 1 minute.
Add chicken stock, coconut milk, fish sauce and sugar; bring to a simmer.
Reduce heat to low; simmer for 10 minutes.
Add chicken to pan; cook 1 minute or until thoroughly heated.
Discard lemongrass.
Top with onions, cilantro and juice.
