Preheat the oven to 450 degrees Fahrenheit (232 degrees Celsius). Line a standard-size baking sheet with parchment paper or a baking mat, and set it aside.
In a large mixing bowl, combine the milk and panko breadcrumbs. Allow them to soak for 5 minutes.
Add the pork, egg, garlic, green onion, carrot, soy sauce, ginger, salt, and pepper. Using your hands, mix together just until combined (overworking it will cause the meatballs to be dense).
Shape into meatballs and place them onto the prepared baking sheet. Place in the oven and bake for about 15 minutes, or until cooked throughout (internal temperature of 165 degrees Fahrenheit or 74 degrees Celsius).
Meanwhile, in a small saucepan set over medium-low heat, combine the hoisin, vinegar, garlic, soy sauce, sesame oil, ground ginger, and red pepper flakes. Bring the mixture to a low simmer, then reduce the heat to low and cook for 2-3 minutes.
Place the meatballs in a large bowl and cover them with the sauce.
Toss to coat and serve hot. Garnish with sesame seeds and sliced green onion, if desired, and enjoy!