Preheat oven to 400 degrees F. Line a baking sheet with parchment paper or a silicone baking mat; set aside.
In a large bowl, combine ground beef, sesame oil, breadcrumbs, ginger, red chili flakes, egg, garlic and green onion. Mix until well combined. Roll the mixture into 1 1/4-to-1 1/2-inch meatballs, forming about 18-20 meatballs.
Place meatballs onto prepared baking sheet and bake for 12 minutes, or until all sides are browned and meatballs are cooked through.
To make the hoisin sauce, whisk together hoisin sauce, rice vinegar, soy sauce, sesame oil, garlic and ginger in a small bowl.
Serve meatballs immediately with hoisin sauce, garnished with green onion and sesame seeds.