In a small mixing bowl, whisk together the tamari or soy sauce, beef broth, rice vinegar, brown sugar, and cornstarch until fully combined. Ensure the sugar dissolves completely and the cornstarch is evenly dispersed to create a smooth, flavorful sauce. Set aside.

Heat a large skillet over medium heat and add the ground beef. Season it with salt and black pepper, breaking it apart with a spatula as it cooks. Stir occasionally to achieve even browning and cook until no pink remains. Drain any excess grease if necessary.

Reduce the heat to medium-low and add the minced garlic and grated ginger to the pan. Stir well, allowing the aromatics to release their rich fragrance without burning. This process should take about 2-3 minutes.

Pour the prepared sauce into the skillet, stirring continuously to ensure the beef is thoroughly coated. Let the mixture simmer, allowing the sauce to thicken and develop a rich consistency. This should take approximately 1-2 minutes.

Once the sauce has reached the desired thickness, sprinkle in the sliced green onions. Stir gently and allow them to soften slightly, cooking for an additional 2 minutes to retain their vibrant color and mild crunch.

Serve the Mongolian Ground Beef hot over a bed of fluffy white rice, with optional steamed vegetables on the side for a complete and satisfying meal.