Preheat your oven to 425°F and line a baking sheet with parchment paper.
Add the breadcrumbs, egg, soy sauce, mirin, and sesame oil to a large mixing bowl and stir to combine.
Add the chicken, chopped green onion, and garlic to the bowl. Using wet hands, mix the ingredients until well combined, but avoid overworking the mixture. If the mixture is too dry add 1-2 Tbsp of milk as needed.
Again using wet hands, form into evenly sized meatballs and place them on the lined baking sheet. you should end up with 16-24 meatballs depending on the size.
Bake for 15 to 20 minutes until the meatballs are browned and the internal temperature reaches 170°F.
While the meatballs cook, combine the cornstarch and water in a small bowl to form a slurry. Then add all of the sauce ingredients and the cornstarch slurry to a small saucepan set over medium heat and bring to a simmer. Whisk frequently until the mixture thickens, then remove from the heat.
When the meatballs have finished cooking, transfer them to a large bowl and pour the sauce over top. Toss until the meatballs are well coated, then garnish with scallions and sesame seeds and serve immediately.