Step 1: Add the Meatballs and Mix the Sauce - Place the frozen meatballs directly into a 5 or 6 quart slow cooker. There is no need to thaw them first, which is one of the reasons I appreciate this recipe so much on a busy day. In a medium bowl, whisk together the hoisin sauce, soy sauce, honey, sesame oil, rice vinegar, minced garlic, chili garlic sauce, and brown sugar. Keep whisking until the honey and brown sugar are worked into the sauce and everything looks smooth. You should already smell the garlic and sesame coming together, which is a pretty good preview of where dinner is headed.
Step 2: Coat the Meatballs and Let Them Cook - Pour the sauce evenly over the frozen meatballs, then stir gently so as many meatballs as possible get coated. Cover the slow cooker and cook on LOW for 3 to 4 hours or HIGH for about 1 1/2 to 2 1/2 hours. If you are nearby, give everything a gentle stir once or twice during cooking so the sauce gets redistributed. If you are running errands or chasing kids around the house and forget, dinner will still be fine. The meatballs are already fully cooked, so this stage is about heating them through and giving them time to soak up all that sweet, savory sauce.
Step 3: Thicken the Sauce Until Glossy - About 20 minutes before you are ready to eat, stir the cornstarch and water together in a small bowl until completely smooth. This little mixture is what turns the cooking liquid into the thick, sticky glaze you want clinging to every meatball. Pour it into the slow cooker and stir gently so it gets mixed throughout the sauce. Cover again and cook for another 15 to 20 minutes. As the cornstarch heats, the sauce will begin to tighten up and look noticeably shinier. If you want an especially thick and sticky finish, you can leave the slow cooker uncovered on HIGH during this final stretch.
Step 4: Finish the Meatballs and Serve - Give the meatballs one last gentle stir so every piece gets coated in the thickened sauce. Sprinkle sliced green onions and sesame seeds over the top just before serving. I like bringing the slow cooker right to the table when we are keeping things casual, especially because everybody can scoop out exactly what they want. For dinner, spoon the meatballs and plenty of sauce over hot white rice. For an appetizer, switch the slow cooker to warm and set out toothpicks so people can grab a meatball whenever they walk past.