Combine the beef, green onions, ginger paste, bread crumbs, egg, soy sauce and red pepper flakes in a large mixing bowl. Thoroughly mix.
Use a 1" cookie scoop to form uniform balls, about golf ball sized. Place onto a parchment paper lined baking sheet.
Put baking sheet into oven and bake for 15 minutes, stopping once halfway through to turn.
To prepare the sauce, combine all the ingredients in a small mixing bowl and whisk. Pour mixture into a saucepan, cover, and bring to a boil over medium heat. Stir often. Once boiling, stir once more and turn off the heat.
Add meatballs to the sauce and toss to coat. Garnish with chopped green onions and sesame seeds. Serve with rice and kimchi, if desired.