Make the meatballs:

• Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper or lightly grease it with cooking spray.
• In a large bowl, combine the ground beef, breadcrumbs, soy sauce, sesame oil, brown sugar, ginger, garlic, green onion, black pepper, and red pepper flakes (if using). Mix everything together until well combined.
• Using your hands, form the mixture into 1 to 1.5-inch meatballs and place them on the prepared baking sheet.
• Bake for 15-18 minutes or until the meatballs are browned and cooked through.

Make the sauce:

• While the meatballs are baking, whisk together the soy sauce, sesame oil, brown sugar, rice vinegar, garlic, ginger, and gochujang (if using) in a small saucepan.
• Bring the mixture to a simmer over medium heat and cook for 2-3 minutes, stirring occasionally, until the sauce thickens slightly. If the sauce becomes too thick, add a teaspoon of water to thin it out.

Toss the meatballs in the sauce:

• Once the meatballs are done, remove them from the oven and transfer them to a large bowl. Pour the sauce over the meatballs and gently toss to coat them evenly.

Serve:

• Garnish the meatballs with sesame seeds and chopped cilantro or green onions.
• Serve immediately with steamed rice, noodles, or as an appetizer with toothpicks.