1. Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the chopped yellow onion and cook for 2-3 minutes until it starts to soften and become translucent.

2. Stir in the minced garlic and frozen mixed vegetables. Continue to cook for another 3-4 minutes, stirring frequently, until the vegetables are tender-crisp.

3. Add the diced cooked chicken to the skillet. Cook for 1-2 minutes, stirring to heat the chicken through.

4. Push the chicken and vegetables to one side of the skillet, creating a clear space. Add the cold, day-old rice to the empty side. Break up any clumps with your spatula and let it heat for about 1 minute before stirring it into the chicken and vegetable mixture.

5. Create another clear space in the skillet. Pour the lightly beaten eggs into this space and scramble them quickly with your spatula until they are just set, about 1 minute. Once cooked, break them into smaller pieces and stir them into the rest of the fried rice.

6. Pour the soy sauce and sesame oil over the entire mixture. Sprinkle with ground white pepper. Stir everything together thoroughly, ensuring the rice is evenly coated with the sauces. Cook for another 1-2 minutes, stirring constantly, until everything is well combined and heated through.

7. Remove the skillet from the heat. Garnish generously with sliced green onions before serving.