Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
In a large bowl, combine ground beef, panko breadcrumbs, egg, minced garlic (3 cloves), grated fresh ginger (1 teaspoon), 2 tablespoons soy sauce, hoisin sauce, brown sugar, and crushed red pepper flakes. Mix until just combined.
Form the mixture into 1 1/2-inch meatballs and place them on the prepared baking sheet.
Bake the meatballs for 15 minutes or until cooked through and browned.
While the meatballs bake, prepare the Mongolian sauce. In a medium saucepan, combine 1/2 cup soy sauce, 1/2 cup water, 1/4 cup brown sugar, minced garlic (3 cloves), and grated fresh ginger (1 teaspoon). Bring to a simmer over medium heat.
In a small bowl, whisk together cornstarch and cold water until smooth. Slowly whisk the cornstarch mixture into the simmering sauce. Continue to cook, stirring constantly, until the sauce thickens, about 2-3 minutes.
Remove the sauce from heat.
Once the meatballs are done, transfer them to a large bowl. Pour the thickened Mongolian sauce over the meatballs and toss gently to coat evenly.
Garnish with thinly sliced green onions before serving.
Serve the Mongolian Beef Meatballs hot with steamed rice or noodles.