Substitute butter with olive oil, if desired.
After 10 hours of cooking, reserved liquid can be strained and used to make soup or sauce.
Onions can be cooked longer for a deeper brown and more concentrated flavor, if desired. To reduce further, leave the liquid in and cook, stirring occasionally, until the onions are darker brown and the liquid is almost completely cooked off, 4 to 5 hours.
Try using a Reynolds® slow cooker liner in your slow cooker for easier cleanup.