In a large mixing bowl, combine beaten egg, milk, panko breadcrumbs, onion powder, garlic powder, salt, and pepper. Let rest for 3 minutes.
Add lean ground beef and gently mix by hand until just combined. Shape into uniform 1.5-inch meatballs (around 24 to 28 meatballs).
Arrange meatballs on a rimmed baking sheet and broil on HIGH for 5 to 6 minutes until lightly browned. Transfer meatballs to a 6-quart slow cooker.
In a bowl, whisk together honey, low-sodium soy sauce, hoisin sauce, minced garlic, grated ginger, toasted sesame oil, and cornstarch slurry until well blended.
Pour sauce over the meatballs in the slow cooker, gently tossing to coat each meatball.
Cover with the lid and cook on LOW for 3 to 4 hours (or on HIGH for 2 hours) until meatballs are tender and the sauce is dark, thick, and sticky.
Stir gently to coat in the glaze. Garnish generously with toasted sesame seeds and fresh sliced scallions. Serve hot over steamed jasmine rice or with toothpicks for appetizers.