Combine ground beef, panko breadcrumbs, eggs, soy sauce, minced garlic, ginger, and half the chopped green onions in a large mixing bowl. Mix gently with your hands just until combined, being careful not to overwork the mixture because overhandling creates dense, tough meatballs.

Roll the sticky teriyaki meatballs crowd mixture into 24 balls approximately 1.5 inches in diameter. Place them on a parchment-lined baking sheet as you work, spacing them about an inch apart so they cook evenly.

Heat vegetable oil in a large skillet over medium-high heat for about one minute. Working quickly, brown meatballs in two batches for 4-5 minutes total, rolling them occasionally so all sides develop golden exterior color.

In a separate bowl, whisk together teriyaki sauce, brown sugar, rice vinegar, sesame oil, and red pepper flakes. Mix cornstarch with water separately until no lumps remain, then add to the sauce mixture and stir thoroughly.

Pour the glaze over cooked meatballs in the skillet and toss gently to coat each one completely. Simmer for 8-10 minutes until sauce thickens and reduces by half, stirring occasionally to ensure even coating and prevent sticking.

Transfer your crowd teriyaki meatballs to a serving platter and top with remaining fresh green onions. Let them rest for 2 minutes before serving so the sticky glaze sets slightly and doesn't slide off when guests grab them.