In a large mixing bowl, combine ground beef, breadcrumbs, egg, finely chopped green onions, minced garlic, grated ginger, soy sauce, sesame oil, and gochujang. Mix gently with your hands until just combined, being careful not to overmix.

Using your hands or a cookie scoop, portion the meat mixture into 1 to 1.5-inch meatballs. Place the shaped meatballs on a baking sheet lined with parchment paper.

Heat a tablespoon of oil in a large skillet over medium-high heat. Add the meatballs in a single layer and sear for about 4-5 minutes, turning occasionally until browned all over but not fully cooked.

In a medium bowl, whisk together soy sauce, brown sugar, rice vinegar, and additional gochujang for the glaze. Blend until the brown sugar dissolves completely.

Preheat your oven to 375°F (190°C). Transfer the seared meatballs to a baking dish and pour the prepared glaze evenly over them. Bake for 15-20 minutes until fully cooked.

While the meatballs bake, prepare the spicy mayo dip by mixing mayonnaise and sriracha in a small bowl. Adjust to taste for your desired heat level.

Once the meatballs are out of the oven, plate them, drizzle with remaining glaze, and garnish with sesame seeds and sliced green onions. Serve warm with spicy mayo dip.