Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
Make the meatball mixture: In a large bowl, combine ground beef, panko, egg, minced garlic, grated ginger, soy sauce, gochujang, sesame oil, black pepper, and chopped green onions. Mix with your hands just until combined (don’t overmix).
Shape the meatballs into 1½-inch balls (about 14–18 meatballs) and place them evenly spaced on the prepared baking sheet.
Bake for 18–20 minutes, until cooked through and lightly browned.
While the meatballs bake, start the sauce: In a medium saucepan over medium heat, whisk together ketchup, rice vinegar, soy sauce, honey, mirin, gochujang, minced garlic, and grated ginger. Bring to a gentle simmer, stirring occasionally.
Make a slurry by mixing cornstarch and water in a small bowl until smooth. Whisk the slurry into the simmering sauce and cook 3–4 minutes, until thick and glossy.
Add baked meatballs to the sauce and gently toss to coat. Simmer together 2 minutes so the flavors meld.
Serve hot over steamed rice. Optional: garnish with sesame seeds and extra sliced green onions.