Preheat the oven to 400°F. Line a large sheet pan with parchment paper.
In a large bowl, combine 2 pounds ground beef, ½ cup breadcrumbs, ½ cup grated Parmesan, 2 cloves minced garlic, 1 egg, 1 tablespoon soy sauce, ½ teaspoon salt, ½ teaspoon black pepper, and ½ teaspoon red pepper flakes. Mix gently until just combined.
Form into 1½ inch meatballs and place on one side of the sheet pan.
Toss 4 cups broccoli florets with 2 tablespoons olive oil, salt, and pepper. Spread on the other side of the sheet pan.
Bake for 18 to 20 minutes until the meatballs are cooked through and the broccoli is tender with slightly crisp edges.
While the meatballs bake, combine ½ cup honey, ¼ cup soy sauce, ¼ cup ketchup, 4 cloves minced garlic, 1 tablespoon rice vinegar, and 1 teaspoon sriracha in a small saucepan over medium heat.
In a small bowl, whisk together 1 tablespoon cornstarch and 2 tablespoons water. Stir into the saucepan and cook for 2 to 3 minutes until thickened and glossy.
Remove the sheet pan from the oven. Pour the hot glaze directly over the meatballs and broccoli. Gently toss everything together right on the sheet pan until evenly coated.
Return the pan to the oven and broil for 1 to 2 minutes, just until the glaze sets and becomes shiny. Watch closely to prevent burning.
Sprinkle with sesame seeds and sliced green onions before serving.