Heat the oil in a large skillet over medium heat. Add the onion and cook until softened, about 5 minutes. Add the garlic and ginger and cook for 1 minute.
Add the bell pepper and curry paste. Stir to combine and cook for 2 minutes.
Pour in the coconut milk, chicken stock or water, fish sauce and brown sugar or palm sugar. Bring to a simmer.
Add the chicken cubes and reduce heat to low. Simmer until chicken is cooked through, about 10 minutes.