In a large skillet or wok, heat 2 tablespoons of sesame oil over medium-high heat. Add diced chicken breasts, seasoned with salt and pepper, and sauté for about 5-7 minutes until golden brown and crispy. Transfer chicken to a plate and set aside.

In the same skillet, toss in minced garlic and diced onion. Sauté for about 2 minutes until fragrant and onion is translucent. Add mixed vegetables and stir-fry for another 3-4 minutes until tender.

Introduce the cooked rice to the skillet, breaking up any clumps. Stir-fry for about 5-7 minutes, allowing it to sit undisturbed occasionally for crispy edges.

Push the rice to one side of the skillet and pour in the beaten eggs. Scramble until just set, then mix thoroughly into the fried rice.

Return the chicken to the skillet with soy sauce, oyster sauce (if using), and rice vinegar. Stir together and cook for another 2-3 minutes, adjusting seasoning with salt and pepper as needed.

Serve hot, garnished with chopped green onions and sesame seeds.