Preheat 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, grated ginger, and 2 tablespoons of sliced green onions. Add 2 tablespoons of soy sauce and mix until just combined.
Form the mixture into 16 evenly sized meatballs and place them on the prepared baking sheet.
Bake the meatballs for 12-15 minutes, or until cooked through and browned.
While the meatballs bake, prepare the Mongolian sauce. In a medium bowl, whisk together the remaining 2 tablespoons soy sauce, brown sugar, hoisin sauce, rice vinegar, sesame oil, and red pepper flakes.
In a small bowl, mix cornstarch and water until smooth to create a slurry.
Heat vegetable oil in a large skillet over medium heat. Add the Mongolian sauce mixture and bring to a simmer.
Stir in the cornstarch slurry and cook for 1-2 minutes until the sauce thickens.
Add the baked meatballs to the skillet and gently toss to coat them evenly with the sauce.
Cook for an additional 2 minutes to allow the flavors to meld.
Remove from heat and garnish with the remaining sliced green onions before serving.