Preheat the oven to 375°F (190°C). Line a rimmed baking sheet with parchment paper.
Finely chop the yellow onion and garlic cloves, then place them in a large mixing bowl.
Add the ground beef, panko breadcrumbs, egg, 2 tablespoons sweet chili sauce, oyster sauce, soy sauce, toasted sesame oil, ground ginger, and black pepper.
Gently mix everything together until evenly combined. Avoid excessively working the beef, which can produce dense meatballs.
Wet your hands lightly with water or use a small cookie scoop to portion approximately 1½ tablespoons of mixture for each meatball.
Roll each portion gently between your palms to create a smooth ball. You should have approximately 26 meatballs.
Arrange the meatballs evenly on the prepared baking sheet, leaving a little space between each one.
Bake for 15 minutes.
Meanwhile, prepare the glaze. Combine the sweet chili sauce, ketchup, rice vinegar, oyster sauce, soy sauce, honey, and toasted sesame oil in a small saucepan.
Place over medium-low heat and bring to a gentle simmer, stirring occasionally.
Simmer for approximately 3–5 minutes, until the glaze becomes slightly thicker and syrupy. Remove from the heat.
Take the Sweet Chili Cocktail Meatballs out of the oven after their initial 15 minutes of baking.
Generously brush the meatballs with all of the prepared sweet chili glaze, turning them carefully if needed to coat as much of the surface as possible.
Return the tray to the oven and bake for another 5–7 minutes, or until the meatballs are fully cooked. Ground beef should reach an internal temperature of 160°F (71°C).
Remove from the oven and immediately sprinkle with toasted white sesame seeds.
Serve warm as an appetizer or alongside rice and vegetables for a complete meal.