In a large bowl, combine ground beef, panko breadcrumbs, milk, egg, green onions, garlic, ginger, and soy sauce. Mix until just combined without overmixing.
Shape mixture into 1 to 1.5-inch meatballs and place in the bottom of the slow cooker. They can overlap slightly.
In a medium bowl, whisk together soy sauce, water, brown sugar, honey, rice vinegar, garlic, and ginger until sugar dissolves.
Pour sauce evenly over meatballs in the slow cooker, ensuring all are coated.
Cover and cook on LOW for 4-6 hours or on HIGH for 2-3 hours until meatballs reach 165°F internal temperature.
About 15 minutes before serving, mix cornstarch and cold water in a small bowl until smooth with no lumps.
Stir slurry into slow cooker. Turn heat to HIGH and cook uncovered for 10-15 minutes, stirring occasionally, until sauce thickens to a glaze.
Garnish with sesame seeds and sliced green onions before serving warm.
Notes
For extra flavor, brown meatballs in a skillet for 2-3 minutes before adding to slow cooker.
Use 80/20 ground beef for juiciest results. Ground turkey or pork work as substitutes.
Panko breadcrumbs keep meatballs light and tender compared to regular breadcrumbs.
If sauce is too thick, add water. If too thin, cook longer or add more cornstarch slurry.