Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper for easy cleanup.
In a mixing bowl, combine the ground beef, breadcrumbs, green onions, minced garlic, egg, salt, black pepper, and ground ginger. Mix until well combined.
Form the mixture into meatballs, about 1 inch in diameter, and place them on the prepared baking sheet.
Bake the meatballs in the preheated oven for 15-20 minutes, or until they are browned and cooked through.
While the meatballs are baking, prepare the sauce. In a skillet or wok, combine soy sauce, brown sugar, sesame oil, rice vinegar, cornstarch, water, and crushed red pepper flakes if using. Whisk until smooth.
Heat the sauce over medium heat until it begins to thicken, about 5 minutes. Stir frequently to avoid burning.
Once the meatballs are done baking, add them to the skillet with the sauce. Gently toss to coat the meatballs evenly with the sauce.
Simmer for an additional 5 minutes on low heat, allowing the flavors to meld.
Serve the Mongolian meatballs over steamed rice or noodles for a complete meal.