Preheat oven to 350°F. Spray a 9×9-inch baking dish with nonstick cooking spray.
In a large bowl, combine ground pork, ground beef, beaten egg, cheese, parsley, salt, and pepper. Mix together until just combined.
In a medium bowl, combine the milk, breadcrumbs, garlic, and onion.
Add milk mixture to the meat mixture being careful not to overwork ingredients.
Greasing hands before starting, form the mixture into desired-size meatballs. (We used a heaping 2-tablespoon scoop for each meatball.) Place into the prepared baking dish.
Place in preheated oven and roast for about 20 minutes (or until the meatballs are crisp on the outside).
While meatballs are cooking, prepare teriyaki sauce.
In a saucepan over medium heat, combine soy sauce, rice vinegar, brown sugar, garlic, and ginger. Whisk ingredients together and cook for 2 minutes or until the brown sugar has dissolved.
Reduce heat to low.
In a small bowl, whisk together cornstarch and water. Pour into the saucepan and simmer until thickened. The sauce should coat the back of a spoon.
Remove the meatballs from the oven. Carefully discard most of the grease.
Pour the sauce over the cooked meatballs. Cover the dish with foil and return it to the oven to bake for an additional 20-25 minutes (or until the meatballs are cooked through – 165°F on a meat thermometer).
Garnish with green onions and sesame seeds.