Place 2 (18-ounce) bags frozen meatballs, 1 (12-ounce) bottle of chili sauce, and 1 (12-ounce) jar of Concord grape jelly in a 5-quart or larger slow cooker. Stir to combine and coat the meatballs with sauce.

Cover and cook until the meatballs are heated through and the sauce thickens slightly, 2 to 2 ½ hours on the HIGH setting or 3 to 4 hours on the LOW setting. For a thicker sauce, during the last 30 minutes cook time, uncover the slow cooker and set the heat to high if needed; stir occasionally.

If serving from the slower cooker, turn to the warm setting. Otherwise, transfer the meatballs to a serving dish. Garnish with finely chopped fresh parsley leaves if desired.