Pour chicken stock into a large saucepan. Stir in chilli 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 frypan over medium-high heat. Stir in cubed chicken and cook until no longer pink in the centre; 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 saucepan from the heat and slowly stir in the whisked egg then gently stir in the parsley. Serve immediately.