Place sliced onions into large slow cooker. Add melted butter.
Cover and cook on low for 9 hours. Stir every 2-3 hours.
Crack the lid to allow liquid to evaporate. Increase heat to high and cook 2 hours or until liquid is evaporated. Stir more frequently during this time.
Add salt and sugar the last 10 minutes.
Transfer to airtight container using slotted spoon. Refrigerate.
Notes
Cooking time may vary depending on slow cooker. Yields 2 CupsAdapted from TheKitchn