Heat a large skillet over medium-high heat.
Add the ground beef and cook for approximately 8–10 minutes, breaking it into small crumbles with a wooden spoon or spatula.
Continue cooking until the beef is thoroughly browned and no pink remains. Carefully spoon off excess fat if necessary.
While the beef cooks, prepare the Mongolian beef sauce. Add the brown sugar, water, low-sodium soy sauce, cornstarch, grated ginger, minced garlic, and optional crushed red pepper flakes to a medium bowl.
Whisk thoroughly until the cornstarch has dissolved and the sauce is smooth.
Add the scallion pieces to the browned beef.
Give the sauce another quick whisk, then pour it into the skillet.
Stir the Mongolian Ground Beef continuously as the sauce begins to simmer and thicken.
Cook for approximately 2–3 minutes, until the sauce becomes glossy and evenly coats the ground beef.
Taste and adjust the seasoning if necessary. Because soy sauce is already salty, additional salt may not be needed.
Remove from the heat and serve immediately over steamed rice or noodles.