Start the sauce: Add grape jelly, chili sauce, and apple cider vinegar to the slow cooker. Cover and preheat on HIGH while you make the meatballs so the glaze loosens and warms.
Mix the meatballs: In a large bowl, gently combine the ground beef, grated onion, Italian seasoning, Worcestershire, garlic powder, salt, and black pepper until just blended (avoid overmixing).
Portion and chill: Scoop into 1 to 1 1/4-inch balls (about 36–40 pieces) and set on a parchment-lined sheet. Refrigerate 10 minutes to help them hold their shape.
Broil to brown: Position an oven rack 6 inches from the broiler and set broiler to high. Arrange meatballs on a rimmed sheet fitted with a wire rack and broil 6–8 minutes, rotating the pan once, until the tops are well browned but not fully cooked through.
Slow cook: Stir the warmed sauce, add the browned meatballs, and turn to coat. Cook on HIGH for 1 1/2 to 2 hours, stirring once halfway through, until the centers reach 160°F and the sauce is glossy and clings to the meatballs. Switch to WARM for serving.
Serve: Keep in the slow cooker on WARM for parties, or transfer to a platter with toothpicks. Spoon extra sauce over the top.