Pour chicken broth into a large pot. Stir in hot pepper flakes, vinegar, soy sauce, lemon grass, fish sauce, black pepper, garlic powder, and onion. Bring to a boil over high heat, then reduce heat to medium-low and keep at a simmer.
Meanwhile, heat olive oil in a skillet over medium-high heat. Stir in cubed chicken, and cook until no longer pink in the center, about 5 minutes. Stir chicken into the simmering soup along with the cabbage and bean sprouts. Simmer until the cabbage is tender, about 10 minutes.
Remove the pot from the heat, and slowly stir in the beaten egg, then gently stir in the chopped parsley. Serve immediately.
