Start by preparing all your ingredients. Make sure the rice is cooled and separate any clumps.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced beef to the hot skillet, season with salt and pepper, and stir-fry until cooked through, about 3-4 minutes. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of oil. Add the minced garlic and diced onion, stir-frying for about 1-2 minutes until the onions are translucent.
Add the diced bell peppers and frozen peas and carrots to the skillet. Cook for an additional 2-3 minutes until the vegetables are tender.
Add the cooked rice to the skillet, breaking up any clumps. Stir-fry for another 2-3 minutes to heat through.
Return the cooked beef to the skillet. Add the soy sauce, oyster sauce (if using), sesame oil, and mix everything well. Stir-fry the mixture for another 1-2 minutes to combine flavors.
Remove from heat and stir in the chopped green onions. Adjust seasoning with additional salt and pepper if needed.
Serve hot and enjoy your Beef Fried Rice!