Prepare all ingredients by slicing the beef, dicing the onion, and mincing the garlic.

Heat the vegetable oil in a wok over medium-high heat.

Add the sliced beef and stir-fry until browned, about 3-4 minutes.

Add the onion and garlic, stir-frying for another 2 minutes until fragrant.

Add the mixed vegetables and cook for 3-4 minutes until tender.

Stir in the cooked rice, soy sauce, oyster sauce, sesame oil, and black pepper. Mix well and cook for an additional 3-5 minutes.

Garnish with sliced green onions before serving.