Preheat the oven to 375℉ (190℃) and line a sheet tray with parchment paper.
In a large bowl, add the egg, hoisin, soy sauce, garlic, ginger and scallions. Stir with a fork. Then stir in the panko.
Add the ground beef (the texture is better if you tear it into small pieces as you add to the bowl). Mix together, by hand, until combined.
Using a 1-tablespoon ice cream scoop, scoop the mixture onto the prepared sheet pan. Roll between your palms to form smooth balls.
Bake until just cooked through, 7 to 10 minutes (do not overbake!!).
For the sauce
In a 3.5-qt braiser or large saute pan, whisk together the brown sugar, soy sauce, hoisin, sesame oil, garlic, ginger, and red pepper flakes.
Bring to a simmer and cook until the sugar is dissolved and sauce is slightly thickened, 1 to 2 minutes.
In a small liquid measuring cup, combine the cornstarch and water. Stir this into the simmering sauce.
Bring the pan back to a simmer and cook until thick and glazey, 2 to 3 minutes. Add the cooked meatballs toss to coat. Sprinkle with more scallions and red pepper flakes.
Notes
Do not overbake the meatballs! This is the worst thing that can happen. So keep an eye on your oven and start checking at 5 minutes.