Place a rack in top third of oven; preheat to 400°. Grease a baking sheet with cooking spray. In a large bowl, combine onion, eggs, beef, bread crumbs, scallions, and salt. Using your hands, mix to combine; try to avoid overmixing.
Form beef mixture into 16 to 18 meatballs about 1 ½" diameter. Arrange on prepared sheet.
Bake meatballs until browned and cooked through, 15 to 20 minutes.
Meanwhile, in a large high-sided skillet over medium heat, bring pineapple chunks and juice, BBQ sauce, garlic, vinegar, soy sauce, and ginger to a simmer. Reduce heat to medium-low and simmer, stirring occasionally, until sauce thickens enough to coat the back of a spoon, about 8 minutes.
Transfer meatballs to sauce and stir until well coated. Top with scallions.
