1. Prepare the Meatball Mixture: In a large mixing bowl, combine the ground chicken, breadcrumbs, egg, finely chopped yellow onion, 2 minced garlic cloves (for meatballs), 1 tablespoon soy sauce (for meatballs), milk, salt, and black pepper. Use your hands to gently mix until all ingredients are just combined. Be careful not to overmix, as this can lead to tough meatballs.

2. Form the Meatballs: Lightly dampen your hands with water to prevent sticking. Roll the mixture into 1-inch to 1.5-inch meatballs. Aim for uniform size so they cook evenly. You should get about 20-24 meatballs from one pound of ground chicken.

3. Cook the Meatballs: Heat 1 tablespoon of olive oil or vegetable oil in a large skillet or non-stick pan over medium-high heat. Once hot, carefully place the meatballs into the pan, ensuring not to overcrowd it. You may need to cook them in two batches. Brown the meatballs on all sides, turning them occasionally, until they are golden brown and mostly cooked through, about 8-10 minutes. Remove the cooked meatballs from the pan and set them aside on a plate.

4. Prepare the Honey Garlic Sauce: In the same skillet (no need to clean it), reduce the heat to medium. Add the honey, 1/4 cup soy sauce (for sauce), rice vinegar, sesame oil, 2 minced garlic cloves (for sauce), and grated fresh ginger (if using). Whisk well to combine and bring the mixture to a gentle simmer.

5. Thicken the Sauce: In a small bowl, whisk together the cornstarch and water until smooth. Pour this cornstarch slurry into the simmering sauce, whisking constantly. Continue to whisk and cook for 1-2 minutes, until the sauce thickens to a glossy, syrupy consistency that coats the back of a spoon.

6. Combine Meatballs and Sauce: Return the cooked meatballs to the skillet, gently tossing them in the thickened honey garlic sauce until they are fully coated. Ensure the meatballs are heated through and thoroughly glazed.

7. Serve and Garnish: Transfer the honey garlic chicken meatballs to a serving platter. Garnish with a sprinkle of sesame seeds and sliced green onions, if desired, for an extra touch of flavor and visual appeal. Serve immediately.