Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the chopped onion and cook for 2-3 minutes until translucent.
Stir in the minced garlic and cook for an additional 1 minute.
Add the mixed vegetables and cook until they are tender-crisp, about 3-4 minutes.
Push the vegetables to one side of the skillet or wok.
Pour the beaten eggs into the empty side and scramble them until cooked through. Mix with the vegetables once done.
If using, add the cooked chicken, shrimp, or tofu to the skillet and stir to combine.
Add the cold rice to the skillet, breaking up any clumps with a spatula.
Pour in the soy sauce and oyster sauce (if using). Stir well to evenly coat the rice and heat through.
Drizzle with sesame oil and season with salt and pepper to taste.
Stir everything together until well combined and heated through.
Garnish with chopped green onions or cilantro, if desired.
Enjoy your meal!