Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
In a mixing bowl, combine the ground beef, breadcrumbs, minced onion, minced garlic, egg, salt, and black pepper. Mix until just combined; be careful not to overwork the mixture.
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 20 minutes or until browned and cooked through. An internal temperature of 160°F (70°C) is recommended for beef.
While the meatballs are baking, prepare the sauce. In a non-stick skillet over medium heat, add honey, soy sauce, rice vinegar, sesame oil, and red pepper flakes (if using). Stir until the mixture is well combined and begins to bubble.
Once the meatballs are done baking, transfer them to the skillet with the sauce and toss to coat the meatballs evenly. Cook for an additional 5 minutes, allowing the flavors to meld.
Remove from heat and garnish with chopped green onions before serving.