To make cornstarch (corn flour) slurry, combine together cornstarch (corn flour) and chicken broth or water. Set aside.
Heat oil in a large saucepan over medium-high heat.
Add garlic, ginger and green chilies. Sauté, stirring occasionally, for 2 minutes or until fragrant.
Add vegetables. Sauté, stirring occasionally, for about 2-3 minutes or until tender.
Add chicken. Sauté, stirring occasionally, for about 2 minutes.
Add chicken broth (stock), ground white pepper, soy sauce, white vinegar and chili sauce. Stir to combine.
Bring to a boil. Simmer for about 2 minutes.
Stir in cornstarch (corn flour) slurry.
Add in egg, stirring, gradually into the soup mixture.
Simmer for about 5-6 minutes or until thick.
Add chopped spring onions or scallions. Simmer for about 2 minutes.