Make the sauce base. In a bowl, whisk together honey, soy sauce, ketchup, vinegar, garlic, ginger, and a pinch of red pepper flakes if you like heat. Aim for a smooth, pourable mixture.
Add meatballs to the slow cooker. Place the frozen meatballs in an even layer.
Pour the sauce over the top and toss to coat so the sauce touches as many meatballs as possible.
Cook low and slow. Cover and cook on Low for 3–4 hours or on High for 2 hours, until the meatballs are heated through and the sauce is bubbling.
Thicken the sauce. In a small cup, mix cornstarch and water to make a slurry. Stir it into the slow cooker. Cover and cook for another 10–15 minutes until the sauce is glossy and slightly thick.
Finish and serve. Sprinkle with sesame seeds and green onions.
Serve as an appetizer with toothpicks or over rice or noodles for a full meal.