Prepare the Rice: Ensure your rice is chilled, as cold rice helps to prevent sticking and clumping. If freshly cooked, spread it on a baking sheet and let it cool in the fridge.
Cook the Vegetables: In a large skillet or wok, heat 15 milliliters of vegetable oil over medium-high heat. Add the chopped onion and garlic, sautéing until fragrant and the onion is translucent, about 2-3 minutes.
Add the Mixed Vegetables: Stir in the frozen mixed vegetables and cook for another 3-4 minutes until they are heated through and slightly tender.
Add Bean Sprouts: Add the bean sprouts to the skillet and cook for an additional 2 minutes until they are tender but still crisp.
Scramble the Eggs: Push the vegetables to one side of the skillet and add the remaining 15 milliliters of oil. Pour the beaten eggs into the empty side and scramble until fully cooked. Mix the scrambled eggs with the vegetables.
Add the Rice: Add the chilled rice to the skillet, breaking up any clumps with a spatula. Stir-fry for 5-7 minutes until the rice is heated through and starting to get a bit crispy.
Season the Fried Rice: Pour the soy sauce and oyster sauce (if using) over the rice, mixing thoroughly to ensure everything is evenly coated. Season with salt and pepper to taste.
Finish and Serve: Stir in the sliced green onions and cook for another minute. Serve hot.