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

Add diced sirloin steak and cook until browned on all sides.

Season with salt and black pepper as desired. Remove the steak from the skillet and set it aside.

In the same skillet, add more oil if needed, then sauté the garlic and ginger until fragrant.

Add the mixed vegetables and onion, cooking until the vegetables are tender yet still crisp.

Create a space in the center of the bottom of the skillet and add the eggs. Cook, stirring and breaking into pieces, until cooked through. Then mix with the vegetables in the skillet.

Push the vegetables and egg mixture to one side of the skillet and add the chilled rice on the other side. Break up any clumps and stir-fry the rice until it's heated through and slightly crispy.

Return the cooked steak to the skillet, and combine it with the rice and vegetables.

Pour the soy sauce, oyster sauce, and sesame oil over the mixture. Stir well to ensure an even distribution of flavors.

Season with salt and pepper to taste, and continue cooking for an additional 2-3 minutes, allowing the flavors to come together.

Garnish with chopped green onions before serving.