Heat the vegetable oil in a large pot over medium heat.

Add the onion and cook for 2-3 minutes until softened.

Stir in the garlic and ginger, cooking for an additional 1 minute until fragrant.

Add the red curry paste and stir for 1-2 minutes to release its flavors.

Pour in the chicken broth and coconut milk, stirring to combine.

Bring the mixture to a gentle boil, then reduce the heat to a simmer.

Add the cooked chicken, mushrooms, and red bell pepper to the pot.

Let the soup simmer for 10 minutes, allowing the flavors to meld together.

Stir in the fish sauce, lime juice, and brown sugar, adjusting to taste if necessary.

Remove the pot from heat and stir in the chopped cilantro.

Serve the soup hot, garnished with green onion, additional cilantro, and red chili slices if desired.