Heat the oil in a large skillet or Dutch oven over medium-high heat. Add the ground beef, breaking it up with a wooden spoon. Season lightly with salt and pepper. Cook until the beef is fully browned and no pink remains. Drain off any excess fat from the pan.

Reduce the heat to medium. Push the beef to one side of the pan and add the diced onion to the empty space. Sauté for about 3–4 minutes until the onions soften and turn translucent. Add the minced garlic and grated ginger. Sauté for just 60 seconds.

Pour the soy sauce directly over the meat. Sprinkle in the brown sugar and black pepper. Stir everything together well, allowing the mixture to bubble for about 2 minutes.

Add the frozen mixed vegetables directly to the pan. Stir and cover the skillet for 3–5 minutes, allowing the steam to cook the vegetables until tender-crisp.

Pour the cornstarch slurry into the center of the mixture. Stir continuously for 1–2 minutes until the sauce thickens into a glossy consistency.

Add the pre-cooked rice and fold it gently into the beef and sauce mixture until evenly coated and heated through.

Taste the mixture and adjust seasoning as needed. Garnish with sesame seeds or green onions if desired. Ladle generous portions into bowls and serve.