In a large bowl, combine the ground beef, breadcrumbs, milk, egg, minced garlic, and black pepper. Mix until well combined.
Shape the mixture into small meatballs, about 1 inch in diameter, and place them in the slow cooker.
In a separate bowl, whisk together the soy sauce, brown sugar, honey, rice vinegar, grated ginger, cornstarch, and water until smooth.
Pour the teriyaki sauce over the meatballs in the slow cooker, ensuring they are evenly coated.
Cover and cook on low for 4-6 hours, or until the meatballs are cooked through and tender.
Once cooked, gently stir the meatballs to ensure they are well coated in the sauce.
Serve the meatballs hot, garnished with chopped green onions and sesame seeds if desired.

Variations & Tips
For a lighter version, you can use ground turkey or chicken instead of beef. If you have picky eaters, consider adding finely chopped vegetables like carrots or bell peppers to the meatball mixture for added nutrition. For a spicier kick, add a teaspoon of sriracha or red pepper flakes to the sauce. You can also make these meatballs gluten-free by using gluten-free breadcrumbs and tamari instead of soy sauce.