In a large bowl, combine ground beef, breadcrumbs, green onions, egg, soy sauce, garlic, ginger, and sesame oil. Mix until well combined, but do not overmix.
Shape the mixture into 1-inch meatballs and place them on a baking sheet lined with parchment paper, or in a parchment-lined baking dish.
Preheat the oven to 400°F and bake the meatballs for 10 minutes or until they are cooked through.
In a saucepan, combine hoisin sauce, rice vinegar, honey, and hot sauce to taste. Heat over medium heat, stirring until the sauce simmers.
Make a slurry with the water and cornstarch, then gradually whisk it into the sauce. Allow the sauce to thicken.
Add the baked meatballs to the saucepan and gently toss them with the sauce until they are well coated.
Serve the meatballs hot, garnished with additional green onions if desired.