Rinse the spare ribs with cold water and remove any excess fat. If desired, line the bottom of the crockpot with onion rings to prevent sticking. Place the ribs in the crockpot.
In a bowl, mix the chopped onion, melted butter, brown sugar, Worcestershire sauce, paprika powder, pepper, mustard, pressed garlic clove, ketchup, water, and dark balsamic vinegar. Add chili powder or flakes if desired.
Pour the sauce mixture over the ribs.
Cover the crockpot and cook on LOW for 9 to 10 hours. After about 8 hours, start checking the ribs. They are done when the meat is very tender and pulls easily away from the bone.