In a mixing bowl, combine the tamari or soy sauce, beef broth, rice vinegar, brown sugar, and cornstarch. Whisk these ingredients together until fully incorporated, ensuring the sugar and cornstarch dissolve completely. Set this mixture aside.

In a large skillet, cook the ground beef over medium heat. During this process, season the beef with salt and pepper, ensuring it browns uniformly.

Once the beef has browned, reduce the skillet's heat to medium-low. At this point, introduce the garlic and ginger to the pan. Allow these ingredients to sauté, ensuring they release their fragrances without burning. This should take approximately 2-3 minutes.

Carefully introduce the previously prepared sauce into the skillet, ensuring it integrates with the beef mixture. Stir the contents to combine and allow the mixture to simmer. As it does, the sauce should begin to thicken. This will take approximately 1-2 minutes.

Once the sauce has achieved the desired consistency, incorporate the sliced green onions, allowing them to cook just long enough to soften slightly, which should take about 2 minutes.

To serve, ladle the Mongolian Ground Beef over a bed of cooked white rice. Accompany with steamed vegetables if desired.