Preheat your oven to 350°F (175°C).

In a large skillet, heat the vegetable oil over medium-high heat.

Add the ground beef and cook until browned, breaking it up with a spoon as it cooks.

Add the diced onion and minced garlic to the skillet and sauté until the onion is translucent.

Stir in the mixed vegetables and cook for another 2-3 minutes.

Add the cooked rice to the skillet, breaking up any clumps.

Pour in the soy sauce and oyster sauce, stirring well to combine all the ingredients.

Season with salt and pepper to taste.

Transfer the mixture to a baking dish and spread it out evenly.

Bake in the preheated oven for 15-20 minutes, until the top is slightly crispy.

Remove from the oven and let it cool for a few minutes.

Garnish with sliced green onions before serving.

For a vegetarian version, substitute the ground beef with crumbled tofu or tempeh. You can also add a scrambled egg for extra protein and flavor. If you prefer a spicier dish, add a teaspoon of chili paste or a dash of hot sauce. For a gluten-free option, ensure that your soy sauce and oyster sauce are gluten-free. Feel free to experiment with different vegetables, such as bell peppers or broccoli, to suit your taste.