Preheat oven to 400 degrees F. Line a large baking sheet with parchment paper; set aside.
In a large mixing bowl, combine ground beef, onion, breadcrumbs, garlic powder, and soy sauce until well combined.
Form mixture into 24 equal sized meatballs (approx. 34 grams each). Place meatballs on prepared pan and spray the top with cooking spray then bake for 15-18 minutes or until cooked through.
Meanwhile, prepare the sauce by whisking together water, brown sugar, oyster sauce, garlic powder, and cornstarch in a saucepan over medium-high heat. Cook, stirring constantly, for 5 minutes or until thickened.
Toss cooked meatballs in sauce and garnish with sesame seeds before serving.