In a large pot, heat the oil over medium heat until it is hot. Pat the chicken pieces dry with a paper towel and season them lightly with salt and pepper. Add them to the hot oil. Let the chicken cook for 1-2 minutes, stirring occasionally. Add the onions and red pepper. Cook for 2-3 minutes, until the onions and peppers are slightly softened.
Add the chicken broth, coconut milk, fish sauce, cayenne pepper, brown sugar and curry powder. Stir to combine. Bring the soup to a simmer and cook, gently, for 10-15 minutes. Stir in the lime juice and cilantro, and spinach if you are using. Add salt and pepper to taste.
3.Serve a ladle or two of the soup over rice and garnish with more cilantro, if desired.