Heat 1 tablespoon of oil in a large pan or wok over medium-high heat.
Add the chicken, season lightly with salt and pepper, and cook until golden and fully cooked. Remove and set aside.
Add the remaining oil to the same pan. Add chopped onions and sauté until soft.
Add garlic and cook for about 30 seconds until fragrant.
Add mixed vegetables and stir-fry for 2–3 minutes until slightly tender.
Add the cooked rice and stir well to break up any clumps.
Return the cooked chicken to the pan and mix everything together.
Add soy sauce, black pepper, and sesame oil (if using). Stir-fry for another 2–3 minutes until well combined.
Taste and adjust seasoning if needed.
Garnish with spring onions and serve hot.